Where can I find a replacement for a Sylvania F40CW/SS fluorescent tube?
Any common T12 bulb is what you need.

You surely have the most common 4' T12 bi-pin type.  They are everywhere. 

You have a wide choice of color temperature (4100k is standard fluorescent color, 5100k is more LED color, 2700K is incandescent color).  

And CRI (color rendering index), nobody will sell you less than 80 anymore, but you can get as high as 98.  Your old tubes were probably 50-60 CRI, the difference will blow you away.  

Tube sizes

The 4-foot bi-pin type bulb is far-and-away the most common.  They come in two primary types that you'll spot in an instant: 


T12 (literally 12/8 inch or 1.5" diameter) - what you have in your hand.  
T8 - notably smaller (8/8" or 1" diam.), will physically fit, but won't work properly.  
T5 - a very small Euro bulb which is an inch too short with too-narrow pins. This is definitely the wrong thing.  


Base types

It's rare, but on occasion you will run into an oddball base type. The 8-foot fluorescent tubes use different styles of base, and some idiot made a 4‘ long version of that - complicating everything. Most of these critters are T12. Just for the record, here are the three base types that are possible: 


normal bi-pin: two plain pins 1/2" apart.  Universal on any tube shorter than 4‘.
monopin: one little nub maybe 1/4” across.  Universal on 8’ style tubes. 
High-Output dual-pin: an odd plastic casing about 1/2" wide shielding two little pins. Used on exotic high-output 8' tubes mostly. 


Consider LED

You can now get "tubes" which are in fact a row of LED lights.  They come in a variety of color temperatures and CRIs (not as good as you can buy in real fluorescent).  Don't just throw a pair in the cart, they take some wiring. 


"Plug-n-Play" types use an existing ballast.  The problem is, it may not be your type of ballast (there are several).  Which will leave you in the painful position of having to change ballasts for a technology that shouldn't even use them.  
"Direct wire" definitely require rewiring the fixture.  But then you get the old ballast out of there for good.