amigakit is the best commercial amiga dealer. he can be expensive so I would look on ebay first....and take your time... it's cheaper that way... but for the roms I would get it from amigakit as well as a copy of os3.9 with emergency disk, thats a good deal.
My opinion.... don't do a tower the 1200 looks great with that small form factor... if you need a more space for cards and such get a 4000....