I'm not sure if the OP is trying to buy a G10 strap in HfH colours or just an 18/20mm strap in general.
There are lots of makers for straps all over the Internet, ranging from £2.99 upwards. My 'issue' watches have 18 and 20mm straps respectively and I've bought them from a variety of sellers in various colours.
Buying the strap from 'A.N. Other' and then simply donating to HfH actually gives HfH the biggest percentage of donation.