I started collecting Disney animation cels in 1977 and had a collection of over 150 at one time. Here are some sites we've enjoyed working with over the years -
https://vegalleries.com/ is based in Sherman Oaks, CA and has a great selection of animation (Disney and others). They also have auctions from time-to-time where you may participate online or 'live' over the phone during the actual auction.
http://www.animationsensations.com/ is based in Washington, DC and also offers a great selection of cels.
Philip Weiss Auctions -
http://www.weissauctions.com/ - is an auction house based on Long Island, NY where you may also participate live during the auctions. They have a wide variety of auctions with the occasional animation pieces as part of the process ,so you'll want to get on their email list for upcoming auctions. Christie's Auctionhouse in NY also has occasional auctions where Animation Art is sold.
Years ago, Disney had The Art Corner store in Tomorrowland (originally opened on Main Street),
Disneyland where cels were sold for a few dollars - these are still available (although no longer at DL) thru some of the internet animation galleries and even ebay - but those pieces may be identified by a gold seal on the back stating the piece was a hand painted animation cel used in a Disney production and released through the Art Corner. The Art Corner was open from 1955 - 1966. For several years at WDW, Mouse Gear (Epcot) sold original animation cels and then limited edition cels. Pleasure Island had the 'Suspended Animation' store where original and limited edition cels were sold. DHS sold animation cels and movie posters at Sid Cahuenga's One-of-a Kind. Sid's is still open (to the left as you enter DHS) but I don't think they offer much in the way of original cels/posters any more. Most of the animation artwork available in DL/WDW are limited edition pieces created for sale but not actually used in a production. The last Disney animated feature length film to use hand-painted cels was The Little Mermaid in 1989. Production backgrounds have still been used in more recent animations, but beginning with Beauty and the Beast (1991) computers have been used to 'paint' cels and even create animated sequences. Some TV productions and commercials still use hand painted cels.
You may still find an occasional animation bargain on ebay, but be cautious as some items are not quite as described as some 'collectors' don't understand what they are really offering. Items described as OPCs (Original Production Cels) may have been from a kit sold at DL/WDW shops and were painted by a guest rather than a Disney Animation artist.
Part of the fun for me was researching the animation. There are some very good reference books about Disney animation by Jeff Lotman (a series of three 'Animation Art' books - 1911 - 1953, 1954 - 1993, and Since 1994 all with photos and description). Another good resource is 'Tomart's Value Guide to Disney Animation Art - with a compliation of over 40 Animation Art auctions with photos. The most extensive collection of Animation from 'Snow White' was put together by Steven H. Ison in Indianapolis, IN who began collecting Snow White pieces back in the early 1960s and even met Adrianna Caselotti (voice of Snow White) at her home - with the gift of a basket of apples. To house his collection, he had a special gallery, which looks like the Dwarf's cottage, constructed on his property.
Enjoy! Animation Art is a wonderful hobby.
Welcome to the DIS!