Dencol is stupid expensive for steel, but they can do a LOT with the machines they have. (We use them for laser cut tabs). If you have them cutting material, their labor prices and time frames make it worth it, but for just buying steel, I would strongly suggest going elsewhere. Your wallet will thank you.
Hinton Steel in Denver is my go to for just sticks and plates. They have the best prices I have found, and if you're going to buy a fair amount of steel, they'll work with you on the price. (I've bought nearly 50K lbs of structural steel in the last 6 months. They know me by name...)
Altitude Steel has a "scrap" pile that they sell by the pound. Pretty cheap if you just need some parts. They have plate, tube, pipe, and other scraps very frequently.
Revelation Steel in Greeley is also a good option, but is further than you're looking at.
And yes, ALRECO for aluminum. Hands down.