Where would one get seat padding ... I have Mk fibreglass seats
I've seen some before but can't find the website

Try JK Composites, not cheap but look

some people use neoprene, some use a camping mat then there are the vinyl covered ones

I used camping mat. Decatlon stores do it in various thickness with a silver surface that looks OK.

I used a black (well, very dark grey) camping mat from ebay for my JK seats

I have this issue, not having a roof, is there a waterproof/non absorbant foam which can be used? Or do I go for go cart style removalable seat pads?