I was surprised how close a metals stockist was to me, for walk-ins at least. If you're buying mild steel - box section or angle, you want the cleanest you can find as you'll need to paint/coat it unless you like rust. Otherwise it's stainless, which carries a premium.
See if there's a local fabricator to you. A small family-run outfit should be easy enough to approach and they might have some offcuts you can have for relatively cheap. I'd imagine for a few quid they'd clean it down ready to paint. Like post above (#19) says, angle can be screwed to worktop from below with wood screws. Box section would typically be fixed from above through the worktop with self-tapping screws.
@Nitefly For the overall expense of buying those legs and getting a mid-beam, you could ask a local fabricators to make you one. If you need a drawing done, PM me all the dimensions and some photos/images and I'll knock you up one on my break. Then you just ask the fabricators for a quote to make the whole thing.
Maybe it was just with mine, but i stuck my head in a welding place near my mechanic and mentioned i wanted something similar to what the OP bought from Etsy, just some square metal legs for a desk and they wanted £140 for a pair which seemed crazy expensive compared to everything i've seen online.