Curtis H. Wilbar Jr. wrote:
The IIsi ROM... but most IIsi's had the rom
soldered on the mainboard.
The IIfx ROM is what you'll need... more common then IIsi ROMs... but
still not
very common to find.
Don't forget there is also Mode/32 which will fix the issue.
http://en.wikipedia.org/wiki/MODE32, so it'll save you some hassle
finding ROMs.