May need to replace shim. 7spd r der. what can I replace it with?
 

Do I need to find another 7spd or will any rear derailer work with what I have. Also will the replacement part need to be specifically designed to work with the old thumb shifters, the type that mount on top of the bar?

Any RD that is Shmino SIS compatable will work, whether it's made by SRAM, SunRace, Falcon, SunTour, or Shimano AS LONG as it has enough 'swing' to span the rear cogset and enough cage length to accomodate your large cog and chain length.

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You didn't give us any information about your bike.

One other thing to look at is how the derailleur attaches to your bike. Most bikes have a tang hanging down from the rear dropout that the derailleur attaches to. On some bikes that part is on the derailleur and extends upward to the rear axle. Try to match what was there before.

Replaced my 7 speed RSX with an 8 speed 105 bought on sale. Run 8 speed chain, works great.

Even though they may be marketed as such, deraillers are not speed-specific. As top506 said, all shimano (and shimano-compatible from other manufacturers) rear deraillers that were designed for indexed shifting have the same cable-pull ratio and thus are compatible with each other. You could use a Shimano 105 rear derailler from 1987, that came with six-speed indexed down-tube shifters and freewheel, with a 10-speed Ultegra drivetrain from 2007. (The only incompatibility here is Shimano 7- and 8-speed Dura-Ace, which has a different cable pull ratio from all other Shimano rear deraillers.)
The only thing that has to match are the speeds of your shifter and your cassette.