Forespar are represented by Nauquip and Aquafax, they both have websites I believe. If the prices are horrific you might be better to import direct.
I assume you need through-hull fitting, seacock and tailpiece? US n.p.t. thread is slightly different to our b.s.p. so don't try to put a Marelon seacock on an existing through-hull, or try to put a UK tailpiece into a Marelon seacock.
I've had Marelon seacocks on a couple of boats and I rate them highly.
Marelon is a trade name for glass filled nylon66. Fittings made from this material are available in bsp sizes from anywhere that sells farm supplies to dairy farmers in the uk. In the south west we have Mole Valley Farmers at Yeovil, Bridgwater and Liskeard - a great source of cheap reliable hull fittings!