I got mine from Aluminium Warehouse, Ecocast (aka cast tooling plate) for the bearing surfaces, 6061 for bits where it wasn't so important like the gantry sides. I also got them to do the majority of the initial cutting to size, as they don't seem to charge a premium for smaller plates.

Although 1000x1000mm is a large amount of aluminium..... £960 for cast Ali that size at 20mm thick from aluminium warehouse... Why do you need so much? I think I spent about £250 on all of the aluminium parts on my build.