It's quite a big and expensive task in GRP, because you have to make a master, then a mould from that, and then the actual spats. So, unless they are serving an aircraft that's already using those big spats, the answer is almost certainly no.
I think I'd be looking to American firms making bushplanes of one sort or another. Or, if you have your own design, talk to one of the UK's several very competent canoe manufacturers.
Alternately, one-offs are much much cheaper to make in light alloy (aluminium to you), and if you can design it, or find somebody competent to, many manufacturers and maintenance / restoration shops probably have the ability to make them for you.