Big place, Scotland!. Might help if you narrow it down.
Try Googling 'aluminium sheet stockists scotland'. Plenty come up that you could try contacting.
Even worth trying ebay, although that might be pricey if you're in an area where they won't do free delivery.
Random example from a very quick look:
www.ebay.co.uk/itm/283528663514?var=584704869620 (check thickness and size, 1250mm x 1000mm is about the size you mention)
Or look for metal fabricators near you. Couple near me (Leicester) I use for various bits of metal & if it's leftover from jobs then it's usually a few £ for the tea fund.