Re:
Is it a thumb type shifter or a trigger type shifter you are after.
i you want to keep the bike stock keep an eye on ebay for 100gs shifters.
My advice would be to forget 100GS which was shite and look for a later used shimano stx or similar rapid fire 7 speed shifter or a used 7 speed DX thumb, thumbie shifter which are far better quality.
I just so happen to have some 7 speed stx you could have for £15 posted, however this has nothing to do with the advice proffered above