How to obtain bacteria samples?
To be honest, you really shouldn't be buying such things if you're not prepared to handle them properly.  If you work with a proper lab, you will be inindated with vendors trying to sell you these and many others.

But to answer your question, if you are looking to purchase many such items ATCC is an excellent resource.  Not only will you have to meet regulatory requirements, but you will also have to deal with a great many MTA's depending on what you are trying to get. 

In response to edit: Salmonella is actually not normally a serious pathogen to a healthy adult.  It is, however, still a human pathogen and should be handled under BSL2 conditions.  So if you have the funds and time to setup a BSL2 lab at your house, by all means go to town.  And I hope you don't live with any children, the elderly, or otherwise immunocompromised individuals.

In response to further edits: 
If you just want to look at things under the scope, then I suggest you get started by making your own homemade culture plates.  If you can get your hands on some Lysogeny Broth (LB), agar, and petri dishes then you can make lb plates like a researcher in a real lab (minus an autoclave but you could use a pressure cooker).  These are items the public could get, but might be harder to come by.

I however will assume that you want to make your own plates from common household ingredients (I used to do this a lot as kid, whatever that says about me).

**

Making your own culture dishes:

**

1) The "plates" themselves.  


I recommend getting the smallest Tupperware like
containers your can find that are microwave safe, dishwasher safe, and
really make a good seal (water tight at the least).  They really
don't have to be big, and you will make your life easier if you buy a
bunch to match.
They need to be super clean.  After running them through your
dishwasher at home, put them all in a boiling pot of water until you
are ready to pour in your "media." A mid-low simmer will work


Note that you won't probably be able to have perfect sterile technique at your house, but you should try to do what you can.

2) Your "Media" or Broth


I highly recommend using pectin as your gelling agent if you can't
get your hands on some agar.  After that comes plain unflavored
gelatin, but I recommend pectin.  You can get it at almost any
grocery store and my recipe is made assuming that.  You can determine
how much you want to make, just scale up the ratios.  I'm going to
list things as rough volume by volume so you won't need a scale.

i) Make the powder first: 


45mL of pectin;
4 bouillon cubes ground as finely as you can (you can get different
types and results depending on if you start with vegetable, beef, or
chicken.  I recommend starting with vegetable.);
125mL of refined white sugar; 
Additives (more on this later)


ii) Mix the powder into 500mL of water


Before you mix in the powder, hear the water to 50-75oC (mid low
heat), but not boiling.  Mix the powder in while stirring until
dissolved.


iii) Bring the mixture up to a boil.
Let it boil for about 2 min, stirring all the while.

iv) Cover the pot and let it cool for 4-5 min.

You by no means want it cool down to room temperature, you just want to get it a little below boiling.  If you are at 55oC or bellow it's definitely time move on.

v) Dip/fill your "plates" 1/3 full of media using clean/sterile utensils and seal while hot but not boiling.

You may have noticed this seems an awful lot like canning, and that's because it is.  Again you want to be as clean as possible in this step and of course try not to burn yourself on anything.

vi) Let your media plates set and chill over night in the refrigerator.  

You really probably only have to wait a few hours if you are anxious to get going.  You are going to want to store the plates that you are not using in the refrigerator anyway.  Also if you made a bunch of plates, let the cool on a counter before you put them in the fridge so that you don't raise the temp of your fridge too much.

Remember the thing about additives?  You can play around with adding different amounts and different types of salts, vitamins, food, or other items to see how they effect the cultures you get to grow.  All kinds of fun there.  First go round, leave them all out.


Now you have your own home made culture plates!  The above recipe was empirically tested by my childhood, and repeated by my son's.  Not all pectin from the store may be equal, this is with Ball Classic (I have no relation to the company).  I'd love to know people's results in trying it, because I might need to re-think how/what I'm doing to get it to work.

Go around your house swabbing things with sterile swabs (you can get these at a drug store), and then streak them out on plate.  While it will be a little different with a swab and homemade plate, the idea is similar to what is done here.

Leave your plate in a warm wet place, but not in direct sunlight.  Ideal temp is going to be around 37, but you can leave them above and below that and still get things to grow.  On top of our dryer is where I leave.

After a day or so you should have colonies you can streak out on a slide and look under a scope.  You will find all kinds of things this way.  If you want a more formal way to put things on a slide, you can check out dry and wet mounting from a Google search.

You are actually quite likely to run into E.coli if you sample enough "areas" this way.  Obviously don't eat or huff the stuff that ends up growing.  Maybe you can post some fun pictures of this if you try it.