When you want Dungeons and Dragons type miniatures...now you will want Reaper miniatures. They really have done a pretty good job of cornering that market. Most are based on the
AD&D dragon forms - although they have also done some more modern looking D&D/Pathfinder dragons. As an upside, Bones Dragons are generally very good...and very cheap.
Schleich does pretty good Disney Dragons (not the cartoonish ones...the give a 4 year old nightmares...save the princess types).
Another excellent source of larger dragons are (were) the McFarlane Dragons. They came prepainted - but had a lot of character and were fairly well detailed. You can still pick them up for around retail price without too much difficulty. WotC did a bunch of them too in their prepaint line (not hard to find...and generally not that expensive).
You can also pick up dragons in a more spotty fashion from other companies...though sizing will become an issue (most will be metal). Iron Winds and Ral Partha Europe have several of the old Ral Partha dragons. Mirilton
SG has a bunch of the old Grenadier dragons. Valiant Enterprises has the old...forget the company...dragons - including one of my favorites...the Greater Dragon (huge):
http://www.veltd.net/zencart/index.php?main_page=product_info&cPath=12&products_id=1524
Even companies like Pegasus get into the act with their Vermithrax Dragon (not really a D&D style dragon...but an excellent dragon all the same).
I'd start with Reaper though, unless you have a specific D&D dragon in mind. If you do - let that be known, and I can probably direct you to who has them.