Would B&Q Warehouse (Queen's Road) sell them? or Wickes? They gear themselves towards trade customers so it would make sense to supply protective clothes too.
Apart from the stall at Moorfoot (and the army surplus shop on London Road) I don't think you're going to find new boilersuits in the city centre.
KEEP AN OPEN MIND, BUT DON'T LET YOUR BRAIN FALL OUT.
People stand in their own shadow and wonder why it's dark.