The A9M0106 3.5" External drive pops up on ebay in the UK occasionally. It is the best drive to have around if you have a mixed collection of old Macs and a IIgs. If you can't buy one, make one from a recycled 800KB drive, ribbon cable and ribbon <--> hacked D-19 male connector.

For the IIgs, assuming that you have enough RAM, you only need one 3.5" drive and one 5.25" drive (and a hacked Disk ][ is good enough). Disk copies on single drive systems using GS/OS are pretty quick *if you have RAM*.