I've used various suppliers and even buy drops and such from random sellers from FB. Picked up some 1-1/4" plate from a guy for dirt cheap recently.
My main supplier is Ryerson Metal Products. I setup an account and use their webservice to create a quote, send it to my sales rep, she reviews it and sometimes I have to special request some products like recently bought some AR400 plate and currently sourcing some A588 weathering steel.
The nice thing is they deliver to me, but I have to have a way to unload. I used to have it delivered to a friend's business and bring over beer, and do some sidework as payment for his time with the forklift, etc. Now that I have a bigger place and a track loader, I order it and have it delivered here.
Literally just had a piece of 5/8" A50 and some CR 16 Ga delivered via tractor trailer. The prices for full sheets and full sticks is cheaper than buying full sheet from Metals Supermarket, but just getting drops or cut pieces is not, and often I have seen an 8 foot cut of DOM be more than if you just bought the whole stick and had it delivered.
I tend to plan and bid side projects based on the cost of a full stick if its something I don't already have a full stick for. Having money ******* in material on hand isn't something I lose sleep over.
The flash rust is from a previous storage situation; the container it is in now has a nice dehumidifier and nothing rusts anymore. Southeast humidity can be a real pain.