I try to use high street bookshops if I can but often neither they nor their warehouse have the book(s) I want. Online I tend to buy first from Wildsounds and then NHBS. I like to buy things from NHBS every now and again as even if it is sometimes a little more as I regard that a price worth paying to have a supplier willing to carry such a large stock of relatively obscure books.
I buy all my New Naturalists from Speedhyhen these days, here's a link to the next one
and the latest Dragonfly book from Smallshire and Swash which I just purchased.
Edit: Now I smell a conspiracy!
Both those links come up as 'invalid' with WH Smith, Foyles and Waterstones as options?