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.
I just did a quick check. Round tube prices have been very stable, except for 3/4 .035. Wicks raised that a bit, so ACS now gets the win. But the good news on the main pieces (1/2", 5/8" 3/4" .035) which make up 50% of the material, prices have barely budged in over a year at either company.
But Wick's almost doubled the square tubing for the canopy (ACS doesn't carry it). That's the one significant bit of steel I had not purchased yet. It will translate int a $70 hit (if not more, by the time I get to buying it). :'(
I order 4130 from both, and find them about six of one and half a dozen of another; both good… I’m east coast, and I think ACS might be a smidgeon faster, but both are pretty quick – I try to keep my lengths to something UPS can deliver… If I need a small piece of something, I’ll order from OnlineMetals, but only for something I need right now (they often use local suppliers and I’ve had 2024 extrusions arrive in less than 24-hours from ordering – don’t know how they do that, but you pay for it…)
Wicks just announced a sale on tubing (see their website), but it is all Chinese. The website says their square tubing is $5/ft, but if you told them you got an email sales announcement, you can get it for $2/ft. PM me if you'd like me to forward the email to you. You have to call for this sale, not purchase online.
It may be perfectly good stuff, but I personally would not touch it for anything structural as I will only trust my behind to known good quality material, although if they had put 1/2x1/2x0.35 on the list (it's not), I would have considered that for the canopy frame. C'est la vie.
Unfortunately, this may mean that there won't be a sale on the good stuff this Fall.
I have a quote for all 4130 material from the materials list in the plans for the stretch version from Wicks for $1600 shipped to California (have to pick it up at a terminal). That is by far the best price I could find.