I believe you can purchase D. castaneoticus from Saurian, which is where I got mine. I have been keeping them for several years, but they've never bred or laid eggs for me. I'm not a breeder, so don't worry about this or particularly encourage it. Compared to other species, my captive group is very shy and not particularly active, over all. They are in my experience, quite frankly, not very interesting, except I give visitors a gold star if they manage to spot one in their two separate vivariums. Other people have reported different experiences about their boldness however, so unless you are a rank beginner with dart frogs, I wouldn't discourage you from trying them if you like them.
There was some sort of fuss a while back about them being illegal, because a Brazilian Government zoo sold some to a zoo in the U.S. with the condition that they would not go into the hobby from this source. Apparently, this zoo did disperse some of their offspring into the hobby, so all such dispersals of these captive bred extra fellows that bred in the zoo, were then considered illegal. You can look up this whole discussion on frognet.com. in the archives. This came after I'd purchased mine from Saurian. I thought the entire reasoning was totally absurd. I wasn't particularly worried that someone from the Idaho Fish and Game Patrol, or some Fed agency was going to confiscate my frogs because they were rather dubiously "illegal." In the first place, they wouldn't have the faculties to identify them, even they even found me in the boonies from whatever clue they got that I had a few of them. If they did find me, they wouldn't be able to find the frogs in their separate habitats, because even I seldom see them, and if they did find them, what would they do with them? Squash them just to prove a point? On the off chance that they had the faculties and know how to keep them alive, where would they send them? Back to Brazil, where their original habitat is being burned for charcoal and short lived agriculture? Back to to zoo that originally dispersed them into the hobby because they bred so successfully there that they had too many?
I think this silly controversy is over, or at least they are available again from Saurian. Good luck, and if you find some, I hope they are more interesting to you than mine have been to me. Experiences do differ, and mine may not be the one to go by. I've read a few posts elsewhere from other people who find them both bold and entertaining.

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Patty
Pahsimeroi, Idaho
4 D. auratus blue
6 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
6 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris