If you are close to higher volume thrift stores this greatly increases your chances.Goodwill superstores are especially helpful.Even so,you have to hit them daily knowing the right time they restock.Yardsales are good places to look and many pop up very close to home.They often offer up suprises you never knew about. These sources,however,require you to settle for what you can find.The great thing about Ebay is that you can choose what you want to get.Though I've found many at local places,Ebay has still yielded the most of my preferred boxes by far.That said,I have found a Lasonic LPC-990,a GE 6035,Mag 8300,and a number of other sought after types at garages sales,local thrifts.
When going to places like Goodwill,always carry an electric cord with you.Usually just an oval,rectangular, or (the more commonly available) closed "U" shaped ended cord will be enough.If you have a figure-8 (or variant) style cord,this will help if your into some '90s JVCs,Panas..I've had luck finding really nice units that have been sitting all day on shelves of high volume stores simply because there was no cord available to test them.My latest find of this type was just a week ago.It was a really cool Fisher PH-W803 surround sound box in near perfect condition and late into the evening!