I made three Santa suits in fairly quick succession, well, in the space of three Xmases. Two were for the primary school 'Santa's grotto' attraction at the winter fundraiser and one was for the school site manager, who does Santa in his own time elsewhere. In all cases I used a fairly firm cotton twill weave. Anything heavier was just too hot to wear, particularly in the confined space of the 'grotto', aka the cubby hole where the photocopier and paper recycling point usually live!
I got the fabric in a shop on North Street (I think) in Chichester, which is where I also got shortish pile fur for the trim and longish pile fur for the beard. Don't live anywhere near there but MiL lives near Havant so was able to make a trip of it during summer visits. I used McCall's M5550.