The limits listed here are each player's initial limits; if you play long enough with an individual player, they'll raise the maximum stakes you can bet for.
Gelrood
Location: Lower City Cantina, Taris
Limit: 40 credits
Gelrood cheats, so you probably shouldn't play him. Simple enough.
Niklos
Location: Upper City Cantina, Taris
Limit: 40 credits
If you win against Niklos enough, he'll eventually accuse you of cheating and won't play anymore.
Sol'aa
Location: Jedi Enclave, Dantooine
Limit: 120 credits
Sol'aa also sells a few cards. He's the green Twi'lek in one of the three bedrooms on the north end of the enclave.
Jolan Aphett
Location: West Central, Manaan
Limit: 200 credits
This guy somehow manages to say "Less talk, more Pazaak" with only two syllables; an impressive Feat. You can abuse his merely average deck for loads of credits if you have the time to save the game between each match.
Gonto Yas
Location: West Ahto, Mercenary Enclave, Manaan
Limit: 200 credits
Kudos
Location: Hunting Lodge, Tatooine
Limit: 75 credits
Kudos' reward for 10 straight wins is bugged, unfortunately; sometimes you apparently get a sigil crystal, other times you'll get an anomalous entry in your journal which has no bearing on the game.
Furko Nellis
Location: Cantina, Tatooine
Limit: Furko will play you for 50, 200, and 500 credits, then refuses to play for money anymore.
Toll Apkar
Location: Cantina, Korriban
Limit: 100 credits