If all you have access to is a big-box craft store like Michaels, Fabricfriend, your best bet is probably to buy one or two sketch pads as your paper source. But if you have an independent art store, or at least a more art-specific chain outlet, you may be able to get a good deal on single large sheets, especially if you skip over the watercolour paper and look for print-making paper. (Printing paper comes in every thickness from onionskin to cover stock, though, so be sure that what you're getting is thick enough and not too absorbent, so ink and paint won't bleed through.)
If there's an art college in your area, you may be able to find good value paper at the campus bookstore or supply store. And sometimes rival art supply stores establish themselves on the periphery of the campus, offering even better deals.