As far as stores go the only option I can think of would be CVS brand (which are not the greatest) The reason I recommend CVS instead of Walgreens is the frequency of sales, and coupons available for extracare card members. I recently burchased 12 bags of 20 ct medium for a total price of less than $23.00 using coupons and discounts. I actually donated the bags to a local nursing home, as I tend to do once or twice a year.
If you are looking for quality try craigslist. The best keywords to search are (briefs, diapers, ABDL, molicare, abena, tena, and dispasable) You will usually find tena, and prevail on craigslist. Every once in awhile you will find better brands though. I just recently bought two cases of molicare. One size large Molicare Super, and one size medium Molicare Comfort Air Active (which I have learned are a bit rare and hard to find. My total price for 180 (2 cases = 3 bags a case. 1 bag contains 30 diapers) was only $120 If you go through 4 diapers a day that means 45 days worth of diapers for $120
Albiet I think your best bet is craigslist if you want quality. CVS if you want cheap and plentiful (but be prepaired to change VERY frequently (6+ times a day)