Easily accessible material for DIY transformer core?
You don't divulge your location so it is hard to gauge what might be present in your local stores, but you can buy toroidal transformer kits with pre-wound primaries (117VAC) and just add the secondary winding. Here is a supplier. 

The volts per turn varies with VA as shown in this table:



You can get cores from old products you might find at the curb on garbage day (or at recycling of electronics days) but most modern products use switching power supplies- microwave ovens being an exception, but even there many newer models are of the "inverter" type.

Hopefully you can do better than this homemade welding transformer: