Figured since I answered your first two questions on this site and you seemed to appreciate it, I would answer this one too.
Before I tell you the cheapest place to get bags, let me try to talk you out of it first.
1. Bags suck. You will have nothing but trouble with them. Expensive trouble.
2. An air system is very heavy. Our car is slow enough as it is. Every pound you add to your car will make it that much slower.
3. Our cars are one of the best handling cars in the world. Professional test drivers at Car and Driver said the only car to handle better than the 7th gen Celica is a Porsche. And that's with stock suspension. Add a good set of coilovers or good shocks and springs and you will improve your handling even better.
Carving up a mountain road with lots of sharp turns is one of the funnest things you can do.
4. If you say you don't care about speed and handling, trust me, you will change your mind soon. Bouncing your car up and down get's old fast.
5. If you say "I just want a show car and not a race car" than a cheap airbag suspension will hurt you at shows.
The guy next to you with a good set of coilovers will get more points than you unless you have an expensive air system.
6. An air system takes up way too much room in your interior and it turns your car from a nice sporty car to a worthless lowrider tank.
Hope this helps. Think about it and then let me know if I have changed your mind. If I did not change your mind, then I will tell you a site that will sell you the cheapest hunk of crap air system you can buy.