without the benefit of being able to afford or get a connect on some premium paints (the ones which are good for steel) I'd suggest sticking with the rusto and trying new adapter and nozzle combos. I don't really use the rusto that requires an adapter since I have a stockpile of the old cans, but even at that.. trying a wide range of nozzles can yield a major difference in the flow and control characteristics of paint.. even the cheaper cans. try some new ones you've never thought to try with american paint before.
short of racking it, or having a connect for it, or just straight up paying retail you got a few options..
1. buy in bulk.. you'll get a much better deal even on the expensive cans if you toss 1k at paint at once
2. start distributing premium cans yourself (this also usually requires buying in bulk)
3. get some sign jobs.
If you get some sign jobs then a part of the process is usually the person doing the job (you) ordering paint. you are the one who decides how much paint it takes to complete your job.
9 times out of 10 in the process of negotiating cost for a project you'll end up agreeing to take less than what you think the job is worth. padding your paint cost estimate is a great way to equalize that factor. so the job should take 40 cans? estimate it at 55 cans for example. in the end you maybe took 3500 for the job instead of the 4500 you bid the job at.. but you're also going to be walking away with some extra paint.
im sure ive missed some angles but.. the sign job thing usually works pretty well. almost nobody wants to pay what a legit sign job costs but they have no clue how much paint it actually takes.. turns out to be a good angle.