Imidazole

Firstly - I cant seem to find race rubber in 225/45-18.

Secondly - Where can I get some lightweight, cheap, basic rims to mount these tires on, so that on autocross day I can just swap the wheels and run?

clyde

By "race rubber" do you mean DOT-R compound tires? For autocrossing, 225/45 probably isn't the size you really want. Look for section widths between 245 and 275 with profiles of 35 and 40. The usual suspects for R comps are (in no particular order):

Toyo RA-1
Kumho Ecsta V710
Kumho Ecsta V710
Hoosier A3S04 (discontinued, but still available in some places at clsoe out pricing)
Hoosier R3S04
Hoosier A3S05
Hankook Ventus Z214

For wheels, if you are running under SCCA like rules in a Stock class where you need to keep to the same wheel dimensions as OE, your best bet is probably a set of OE wheels. They come up for sale pretty regularly in the for sale forum here. Prices vary greatly, but even at the most expensive end, they're still a lot cheaper than most (all?) new aftermarket wheels and lighter weight than most of them. If you go that route, they'll probably come with a set of street tires and possibly even the TPMS sensors...both of which you can resell which may reduce your net cost to the peanuts level. There are lighter wheels out there, but IMO, the cost benefit ratio just isn't there for local level autocrossing.

tuj

Call up tirerack. They have excellent prices and service, and many of their reps are quite knowledgable about r-compound tires for auto-x.