Here are a few of my favorite links:

The model horse sales pages is a collection of vendors selling or looking for models - www.wheelsoff.com/sales

The winners circle always has what's new out first and has a full line of current breyer, stone and others - www.handhwc.com

This site is "THE" Breyer website, except they only sell a few horses off it - www.breyerhorses.com

This is "THE" Peter Stone horse website.  It sells models off it, though they are extremely expensive so I do not buy from them - www.stonehorses.com