Look into the rules and talk with your leader - you likely just need a "shanked" bit vs. something with a solid ported mouthpiece. A "curb" is generally a bit with a shank of any length and that requires a curb chain. It doesn't have to have a solid ported mouthpiece.
If you want to go with something really mild that transitions well with a snaffle, go with a short curved shank and a double jointed mouthpiece. Straight shafts and a single joint (i.e. Tom Thumb style) can be quite severe when not used gently and quite confusing to a horse that is accustomed to a milder bit.
This is the style I would use:
http://www.sstack.com/western_bitscurbs_dutton-bits/dutton-teardrop-dog-bone-snaffle-bit/
Schneider's has a TON of bits to choose from, and is one of the biggest vendors at the AQHA World Show in OKC. You can probably find any bit you want on their site:
http://www.sstack.com/western_bitscurbs . If you want a standard low port curb, but with a little bling to it, you will find lots of options!