I know people swear by lower profile tyres to improve handling, quicker turn in etc etc. But on a vehicle that is taller than it is wide, like a bongo is, I've always reckoned it would fall over before you lose grip even with standard tyres...
But each to their own. If this is what you want to do, start looking in scrappies for Jap cars with five studs (Mazda 626, Ford Probe - I'm guessing), check on an alloy wheel site for the PCD and offset of the donor car, and check that they are compatible with the Bongo. You might find 16 inches, doubt whether there will be many 17 inch steels around though.
Moving up to 16 inches you could come down to 55 profile 215/55 x16, if you can find steel wheels wide enough.
Calculations here:
http://www.miata.net/garage/tirecalc.html