A very useful thread with several suggestions.
I've generally used the regular model-engineering retailers for most, but for larger and heavier sections such as for my workshop's overhead crane parts, used Pulham Steels, in the Dorset village of the name. Their main customers are trade including farmers, but they seel to the DIY customer as it were. I buy and collect complete lengths but cut to either needed lengths or to be simply transportable ("Oh, and the bar-ends too, please! "
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Scrap-yards can yield good finds but you cannot know the steel grades, and the production bar-end with a lovely faced end might prove an absolute what-is-it to machine on a small lathe with conventional tooling.
For sheet-steel work I have used old central-heating boiler panels, server case parts and the like. The thinner sheets are within the capacity of lightweight forming equipment, and its original use shows it of folding quality.
This is beginning to make me feel 'orribly like one of them so-called "influencers". Only they are paid for writing what are unashamedly advertisements. (I assume of course they pay their NI and Income Tax, and insure their bedrooms for business use!).
However…
Toolstation stocks a limited range of architectural metals and a good range of the common ISO-Metric fasteners, circlips, all manner of pressed-steel brackets, tools etc. For non-model metalwork (equipping the workshop, making miniature-railway equipment, traction-engine driving-trolleys, etc.) it's surprising what can be obtained from TS and similar, primarily building-trade, stockists.
I've seen B&Q mentioned on this site. Its basic selection of light aluminium and steel sections is OK for odd tasks and I have bought from there, but the materials are primarily for small household DIY work not high-grade engineering, not cheap, and the quality of the steel can be variable. The canopy poles on my steam-wagon are thin-walled steel tube from B&Q, intended for e.g., wardrobe rails, but they are not supporting any significant load on the wagon. The company used to have a good on-line catalogue… Used to, but it's been re-written to conform with modern retail-chain practice.