I'm looking to get hold of an 8 speed grip-shift shifter for a hand cycle. Do these differ much from similar gear shifters for conventional bikes? Should a regular Shimano 8 speed shifter work?

Thanks

There have been 2 kinds: 1 for a particular Sram RD & 1 that works with Shimano.

Handcycles generally use the same shifters as traditional bicycles. What derailleur do you need it to control?

The brand of your rear derailleur will dictate which shifter will work. SRAM makes two different kinds. One works with SRAM derailleurs and one works with Shimano. Shimano makes a Revo shifter that works with Shimano derailleurs.

Also old Sram grippies work w/ Shimano mechs. Old as in before they had their own derailers.