Yeah, Bill is right, there isn't a store here - that I know of - that sells manuscripts. If you did find a store selling some, it would likely be one or two, and you'd probably have to know who to ask for.
There are a few good bookstores left in town, but not many (record stores must outnumber book stores 3 to 1 now), and none that could spread out a variety of manuscripts in front of you to choose from.
I guess it's not surprising that people expect such a place to exist. I must get 50 emails a year asking, "Where can I buy some of Bukowski's art?" As if there's a single source. There isn't, for the record, so as Bill said, you'll have better luck trawling eBay or waiting for the next auction.