With darts and mantellas you will need to raise your own food (fruit flies). As long as you have a heavily planted, humid terrarium, darts aren't too hard to keep. Your best beginner dart is probably Dendrobates leucomelas, AKA the bumble bee dart frog. D. auratus and D. tinctorius are good too, but females can be quite terratorial with these two species.

I have never seen sirens for sale. Amphiuma, yes, but not sirens. Just FYI, Siren lacertina is known to include vegetable mater in its diet.