flea markets, garage sales, and thrift stores. I have picked up some 12-15v 1+ amp wallwarts for a dollar or two at them. I recommend "stocking" the following: 12v, 9v, 6V, 4.5v, 3v. 500ma is a good starting current amount with the larger voltage ones being higher current. almost all walwarts have center pin + on them and over half of them use the same standard plug size. You can use a larger ma walwart on anything as long as the voltage is identical and in some cases if needed you can use a smaller voltage walwart that puts out higher current and it may work too, like using a 6v 700ma walwart on something needing 9v 500ma but YMMV as sometimes you won't get full output (boomboxes on high). Things that use batteries will sometimes run at less voltage fine off a walwart because they are designed to run alkalines dead to 1v each so a 6 cell device would start at 9v and stop running about 5-6v because the batteries have no power left in them at that level.