What I care mostly about is the condition of what I buy. I want my posters in mint condition, or as close to mint as the passing and damage of time will allow. So Emovie is always a great source. Heritage is okay, although their grading is absolutely horrendous since Grey Smith jumped ship so you must really check the pictures and cross fingers. Propstore is good, this time thanks to the same Grey Smith. Ebay is always an option, but 90% of sellers have no idea how to pack and ship posters. Dale Dilts, who is a member here, is pretty much the only safe source on Ebay for modern posters. Since you live in the UK, there are a few good sellers that I trust for quads, but it took me quite a few attempt to find them. Most sellers do not understand what mint means. Some collectors don't care, and if it is your case (you did mention buying bulk lots), then the world is your oyster and you'll be fine. There are a few boutique out there, but their pricing are all over the place and you could end up spending 10x what an item is actually worth. It wouldn't be a service to a new collector to name them on a public forum, but you can do your homework. Good luck. It's a fun hobby.
T