Hi Lee,
Do you have a 'B & Q Warehouse' in your area?
If you do, they have a good selection of 'Rolled' (flat) steel, as well as various diameter Steel Bar and Tube, in both 1 m and 2 m lengths.
They also have Corner (angle) section, and a selection of Aluminium pieces in the same sizes (although these are a little more expensive).
It's usually in the 'Hardware' section (with the Nuts, Bolts, and Screws), and vary in price, depending on what you need.
Foe example, a 1 m length of 20 mm x 4 mm Cold Rolled (flat) steel is about ?4.50, and this will make most of the flat parts shown in the build manual.
Just another place to source parts I thought you might find useful.
Keep Busy,
Regards,
Mark.