Wick's and Aircraft Spruce (just up the road from you in Corona). If you are not in a hurry, both will likely have a sale around September/October. Last year it was 15%. Get on Wick's mailing list to get notification of their sale.
As of last September, Wick's had the best price for almost everything except 5/8" .035 tubing, bushing stock, and some sheet. Fortunately, you can truck right up to ACS for your 10 foot longeron pieces (which happen to be 5/8 .035) and still get them (substantially) cheaper than Wicks. Also fortunately, prices have tended to be petty stable the last few years. If you are trying to buy all at once, be prepared to part with a bit over a kilobuck for all of the steel you will need.
You will pay sales tax at ACS, and UPS charges for Wick's, so these two factors tend to balance out.
Both of them sell quality German tube. And for both best price and quality, save yourself some time and stick with these two suppliers. You will have to use both anyway, neither has quite everything you will need.