There are growing numbers of "Meta-Sites" that can launch you.  While
none of these is completely comprehensive, in combination they provide
tremendous opportunities for exploration.

http://www.bc.edu/bc_org/avp/cas/fnart/Artweb.html
http://sln.fi.edu/tfi/hotlists/museums.html
http://www.tile.net/tile/listserv/art.html
http://www.artdealers.org/
http://204.142.194.96/students/mapuzzo/art/index.htm
http://iserver.iie.org/ai/
http://webcrawler.com/select/visart.23.html
http://sln.fi.edu/tfi/hotlists/museums.html
http://www.icom.org/vlmp/usa.html
http://www.icom.org/vlmp/world.html
http://www.auctions-on-line.com/auction/int_auct/

These sites very in quality, but the general list will provide an
excellent start.

Ted Aiken
Lowe Art Gallery
Graduate Program in Museum Studies
Syracuse University