Try classifieds on this site like Gecko said. But sometimes if you are looking on ebay you need to be creative with keyword searches, or be a litle more diligent in the searches. Its frustrating on eBay sometimes because there are usually hundreds of softboot bindings for sale for every plate binding you may find.
Also, not all plate bindings for sale are being sold by hardbooters. Therefore, sometimes they have no clue what they are selling, and you need to take your time with a search. This may also be adventagious to the savy buyer, becasue he/she may be able to poach a pair at a low price.
Also, what do you need, a 4H pattern (likely) or a 3D Burton pattern?(less likely)
Here are a few on ebay right now that are used:
(The first pair is mine I am selling,[shameless plug], but the rest are not mine)
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7116800237&rd=1
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7116424773&rd=1
these appear to have the 4H discs, but they are the oldest style of Burtion made:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7116799827&rd=1
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7117169419&rd=1
This guy almost always has a brand new pair of Burton plates for sale:
race plate:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7117432681&rd=1
carrier plate:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7116428312&rd=1
rental/demo race plate:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7117434002&rd=1
Here's a another guy with race plate step ins for sale:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=21248&item=7117250250&rd=1
So, that makes 8 pairs of plate bindings currently for sale on eBay, all with a price that is easier to swallow than the new TD's, and 4 of the choices are brand new!!
Just make sure that if you NEED a 4H pattern, that those discs come with the Burton bindings, otherwise, you may need to hunt down a pair at a snowboard shop.
I prefer the TD's too, but if you are on a budget, these choices will at least get you out there carving on the hill until you can afford them!
Good Luck!
-Chris