Hi Peter,
Of the 4 you mention, Homestead and Goat Nutrition are pretty reliable as to dates of cultures/rennets, etc., but they are limited for choice, as you say. Orchard Valley can prove expensive in some cases; you have to pick and choose cultures to keep costs reasonable. Since coming to this forum, it looks as though cheesemaker.com might be fine for cultures' quantities and prices. From what I can work out, many of them will ship without ill effects and the cost of postage to the UK isn't that great. The site might work out best for prices for small-scale home producers in the UK.
Other than that, I often make a sludge of rinds or pates of bought cheeses I like and add those to the milk, or culture, or to use as a wash. It's not always reliable, though I've never had complete disasters this way - just some uneven results.