There might be one best price seller for one type of film but even here seller A has the best price for 1 film but seller B beats him if you buy a pack of 5 or 10. If you buy several different films then it is a question of taking an "on balance" view. If you try to buy from several different sellers to get the best price at each then in my experience the extra postage and packing at say 3 different retailers compared to 1 will outweigh the savings.
If you are a HP5+ specialist then check out AgPhotographic. It may be cheaper and will probably deliver the film quicker than 7Dayshop but the latter will deliver free so that needs to be taken into account
You really have to do your own research and then periodically re-test the market for prices