The Rota RBs only come in one offset: +5mm, unless something has changed recently. They will stick way out of the fenders on any Miata. They're cheap and they're wide, but that's about all they've got going for them.
Diamond steelies are a great inexpensive way to go for a '90-'93 Miata. However, they won't clear the brakes on a '94+ Miata if you decide for whatever reason to try them on your other car.
Mark52 on this board has a set of 13" Real Racing wheels on his CSP car. You might PM him for some more info on fitment and what offset he's using.
Panasports are a great choice, but they're definitely not the Team Cheap Bastard option. Pricey (~$1200 a set), built to order, and usually take a few months from order to delivery. You can order them (and find online pricing) through Mid-Atlantic Motorsports.
Edit - One other option I forgot about: Axis/Lenso VPDs. They're a little narrower than you'd want if you were after a National title (13"x7.5"), but they're inexpensive (less than $600 a set), light (around 9lbs each), come in an offset that works fine on a Miata (+20mm), and clear the brakes on all NA Miatas. Goodwin Racing may still have a few sets. When I was contemplating going to CSP two years ago, this was what I'd planned on buying, as it's the only real sub-$1000 option that will clear the '94-'00 brakes.
As for tires... if you're just starting out, I'd probably recommend something like the 235/45-13 Ecsta V700s, 215/50-13 Yokohama A032Rs, or 225/45-13 Hoosier Autocrossers (not A3S04s). If you're looking for a long-lasting tire that's good to learn on, the A032R is probably a good choice. A Kumho Victoracer is also a good choice, but I don't know if they offer a 13" size off the top of my head.
Hoosier A3S04s or Kumho V710s (I believe they're supposed to have a 215/50-13 this coming season) are going to be the most competitive tires, but they probably aren't a good choice to learn on, as they don't tend to last as long as the others I've mentioned.
As for ordering tires, I usually just order my tires through Tire Rack and have them mounted locally. There is a local Discount Tire manager (Todd Eady in Flower Mound) who sponsors some local SCCA stuff and will match Tire Rack prices, but Flower Mound is too far for me to drive to save twenty bucks.
Hope that helps.