You can buy a 'Squirrel' SCSI interface, which fits in the PCMCIA port. These are cheap, but quite slow...
see:
http://www.amiga-hardware.com/squirrel.htmlbut you would still need to boot from a floppy disk
A better idea would be to buy a 2.5" IDE hard disk (laptop hard disk) and fit it inside your A1200 and you can boot Workbench from this (the A1200 has a 40-pin IDE connector inside). If you give me your address [using pmail], I can send you a small one (only 40MB, sorry) for free, but you will need a cable...
edit: Here is a picture of the inside of an A1200, showing a 2.5HD (shamelessly pinched from Jaruzel's site)
http://www.jaruzel.com/amiga/img/main/NewHDD.jpgAnyone help with the cable?