You should check out sites such as pipesandcigars.com and smokingpipes.com Hearth and Home and Cornell & Diehl make some pretty affordable and mostly very good blends, which are usually available in bulk.
I would avoid the 5-pound bags of so-called "pipe tobacco" sold in "tobacco outlets", where cigarette smokers go to buy cigarettes and roll-your-own tobacco - it isn't really pipe tobacco. These mammoth sacks are cheap, but they contain more stems than quality tobacco. I think it's basically crap from the tobacco plants which produce mostly for cigarette companies, swept off their floors, and not good enough enough to make cigarettes with. Stems are mostly water, and they release it all into your bowl for a soggy, nasty smoke when they burn.
Really cheap cigarette-shop "pipe tobacco" also stinks like a wet fart. I spent a few evenings with a friend who rolled her cigarettes with that garbage, and the stench was overwhelming.
You may find that even decent tobacco blenders don't perfectly sort out their stems when producing ribbon-cut blends, but they do it a lot better than those who sell it by the 5-pound bag in cigarette shops. It may behoove you to remove any stem material which you may find while packing your bowl.