nobody makes a TRUE CAI...there's an Injen SRI, and a CA extension, but the extensions are no longer being produced, ergo, you must buy the SRI and then try to find a used extension. BUT the best bang for your buck on the intake is CP-E (
www.cp-e.com, i think), as they make a better SRI, and also have a MAF modulator that piggybacks your ECU to tune your A/F ratio.
JPP Headers are the most reliably common headers. i think they make a y-pipe too. otherwise, there is very little aftermarket support for solara's.
i'm not totally sure on all the exhaust part manufacturers, as i haven't looked into upgrading mine yet (therefore i havent researched extensively). i know that there were some custom y-pipe's made by a member on here (datsrboi), but they were in limited quantities and custom built, and i dont think anyone on here is selling theirs.
i'm sure someone else will help you out on what available exhaust systems there are, i'm just not really sure, and dont want to confuse you
as for the ecu, an Apex-i SAFC II works just fine (i'm told), as you are able to fine tune your A/F ratio to match your mod's. don't buy an ebay 'superchip' or any of that trash, as it is usually a waste of money, and potentially harmful to your engine. also, don't buy a generic ebay air intake, as those are also trash.
if you are serious about modding your ride, be prepared to drop some coinage, as no real performance parts are cheap. otherwise, if you cheap out on mods on the first go-round, then you'll save money until you blow up your engine, and have to drop a couple grand on a new one.
the cheap mods that are effective are jim's torque rod (~$55 if you can find one for sale), TRD FSTB, and TRD RSB (Both 100<$<200 a piece), and none of those actually affect anything having to do directly with your engine parameters.