When you need a legal treatise on a particular subject, your best bet is probably to search your local library catalog. If that doesn't turn up what you need, other good sources include:
If you need to broaden your search, you can do a subject search in WorldCat. You can also search the digital collections on Google Books and/or Hathitrust.
Many older treatises are available on HeinOnline, either in the Legal Classics Library or one of their other collections.
To find a treatise on a given subject for a particular state, check Searching the Law: The States, search the online catalog of a law library in the state (again, see "Libraries" for links) and/or search Indexmaster. If necessary, call a reference librarian in the relevant state (e.g., look in the AALL Directory or use Martindale-Hubbell to look up a large firm with a practice in the relevant area) and ask for the librarian).
Prices: You can generally get the price of a legal treatise from the publisher's Web site, though you might find a better price on Amazon.com, another discount book vendor or through a used book dealer (see "Used Books"). To track the price of a treatise back over several years, use the Legal Information Buyer's Guide & Reference Manual (New England LawPress) by Ken Svengalis. AALL members can search the Price Index for Legal Publications on the AALL Web site.