Sadly, I haven't really found a reliable source for online purchases, outside of Amazon; you just have to know what you're looking for, unfortunately.

Couple good books on the history of Gay Pulp (both with fantastic recommendations):

Queer Pulp https://www.amazon.com/gp/product/0811830209/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Pulp Friction https://www.amazon.com/gp/product/0312252676/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

Of the two, Pulp Friction covers the most history but is a bit "scholarly" and Queer Pulp skews more towards the cover art. But again, both are great resources

And if all else fails and you find yourself in San Francisco, you MUST go to Auto Erotica in the Castro. Pretty good selection of pulp (vast majority of mine has come from there) and it's grouped by price.