Usually you can get some used Deore II or DX 7-speed thumbshifters for $30 or so on eBay, XT 7-speed for $70-ish.
Nothing up at the moment save for pricey NOS XT, but Keep your eyes peeled there.
If you don't mind friction shifting there are some new budget thumbshifter choices like Falcons for $15.
OH, by the way, what rear derailer do you have?