It is quite possible that you can get one at your local auto parts store!
O'Reilly Auto Parts have a selection of common-sized Woodruff keys at prices from $.29 to $2.69. They may be in inventory or available in as little as three hours from a nearby warehouse.
I am sure that other auto parts stores can do the same thing.
Just measure the slot's width and depth, dump all the change in that mayonaisse jar sitting on your dresser into your pockets and rush out in a buying frenzy to the nearest auto parts store! Buy a bunch of similar-sized woodruff keys. Return the ones that do not fit for a refund.
EDIT: as crgm191 mentioned, MAKE SURE THAT YOU PROPERLY TIGHTEN THE CRANK BOLT More G13 and G16 engines get screwed up by improper torquing of the crank bolt!
If you don't have the torque specification, you can find it by downloading the appropriate section of the 1988 Suzuki Samurai Factory Service Manual (FSM). Do a search for fsm at ack's FAQ (the link is in my signature, below) to download a copy.
I hope that this helps!