Most people start their collections with the various "color change" decks - Bicycle Rider Backs in different colors of the rainbow. If that's your thing, great - but most collectors I know just ignore them after a while. In today's custom market, they're pretty tame by comparison to what's available.
Some will hit up the various magic/cards sites that make decks. Among them are:
coterie1902.com
ellusionist.com
theory11.com
dananddave.com
thebluecrown.com
davidblaine.com
bigblindmedia.com (makers of the Karnival decks, among a few others)
penguinmagic.com (not many new designs yet - they're a relative newcomer)
gamblerswarehouse.com (they have a retail site but I forget the name off the top of my head)
collectableplayingcards.com
Some of the more magic-oriented websites will have many designs that are continually reprinted - in particular, Ellusionist, the Blue Crown and Theory11 - in addition to limited edition decks. THESE are the best decks to get as a poker player - they're relatively inexpensive AND can be replaced relatively easily. But if you want stuff that's a little more exotic, there's always the limited runs. You can also find a LOT of new, unique and innovative designs at kickstarter.com - it's a crowdfunding website where artists raise funds directly to pay for printing their design ideas. Be very selective when you're there - some artists use top-notch printing companies while others use budget, bargain-basement print shops that aren't worth the expense.
When in doubt, ask around here - someone's likely going to have the skinny on what's out there.
BTW: welcome to the forum!