On this side of the pond (USA), the main sources are:
Bill McKivor (The Copper Corner; no pictures but issues a quarterly list of offerings; planning to retire soon)
Dr. Gary Sriro (garysriro on eBay; sporadic offerings on eBay; known for likely the most complete collection of Conder varieties ever collected )
Heritage auctions (occasional large collections; otherwise sporadic smaller groupings)
Gary Groll (I have never purchased anything from him, because I find his asking prices to be mostly absurd)
James Ricks at Atlas Numismatics (often aggressive pricing, but the occasional "deal" can be found)
Ernie Latter (cwtguy on eBay; usually has a few for sale in true auctions; some BIN also)
Stephen Oatway (Based in Canada; britannianumismatics on eBay; usually has a few Conders in stock)
Holgate Numismatics (andiepaul03 on eBay; Largely sourced material from Heritage Auctions with price mark ups)
Ancient Galleon, LLC (ancientgalleonllc on eBay; decent selection and variety)