Rare decks....interesting question, because the popular decks arent really that rare. If you are looking for a few OLDER decks that are worth having I can help you. As far as popular goes I could care less, Im a collector 
Start with ebay and look for
Blue Ribbon 323s
Aristocrat 727s
Battle Axe (the old ones)
Bicycle (New Fan Back, Automobile Back, Lotus Back ect.)
Steamboats (plaid and old)
Arrco Tahoes (hands down the BEST DECK EVER MADE I bought a few of these from Richard Kauffman a few years ago)
I could list many many others, but these pop up on ebay from time to time.
The older casino cardsare another story. Jerrys Nuggets arent THAT rare, because I see them on ebay all the time!! Look for..
Desert Inns
Dunes
Las Vegas Club (old stamp Joshua Tree)
Silver Slipper
THESE are the decks to buy in my opinion, but after 20 years what do I know, lol??
Start with ebay and look for
Blue Ribbon 323s
Aristocrat 727s
Battle Axe (the old ones)
Bicycle (New Fan Back, Automobile Back, Lotus Back ect.)
Steamboats (plaid and old)
Arrco Tahoes (hands down the BEST DECK EVER MADE I bought a few of these from Richard Kauffman a few years ago)
I could list many many others, but these pop up on ebay from time to time.
The older casino cardsare another story. Jerrys Nuggets arent THAT rare, because I see them on ebay all the time!! Look for..
Desert Inns
Dunes
Las Vegas Club (old stamp Joshua Tree)
Silver Slipper
THESE are the decks to buy in my opinion, but after 20 years what do I know, lol??