I get my games at the following sources:

1) Thrift shops (lucky in most cases)
2) eBay
3) GOG.com for DOS and some Windows games (pretty good bargain)
4) Steam for DOS and Windows games (cheaper than getting a physical copy at a store since they have sales)

I had purchased about 4-5 games that are in the big boxes that are either on CDs or diskettes and I have big boxes for the games that my dad had purchased between 1992-1996 or so. My plan is to scan those boxes, order them as 8x10's, and buy frames to put them in as a display or in a glass case for the big boxes to preserve them.