"We also have an old desktop that has nothing but dial up capability. " double check. Almost everything from win95 on had a hard wired ethernet port for a cat5 cable. It looks like a phone jack but has more wires. If you are in the same room as the router then you can just plug in. You will see the same jack empty on your router.
A wireless USB stick is $20-30, but will only work for winXP and later. You will not find one for an OS older than winXP. (And truly if the HW is older than XP you are better off not trying to use it. Ignoring the speed of the equipment, the failure rate on disk drives more than 10 years old is high.)