Hi,
You don't necessarily need the 3.1 ROMs to run OS3.1 on a 3.0 system as I'm doing this on Amiga #2 right now. However, should you wish to upgrade to OS3.5/9 then you'll need the ROMs.
I got mine for Amiga #1 from
www.amibench.org, after placing an ad. You'll need A1200 specific ROMs though, so don't be tempted by anything that has come from any other model of Amiga.