The link above is what I would point you to. I'm not sure why I didn't in my original post other than I was at a conference with a crappy internet connection.
Anyway. I really do not like the thinking that the purpose of a curb is to have more bite or be more bit than a true snaffle. Wrong reason to go to a curb and not what they were designed for. Curbs are designed for use on horses advanced in training so that you can give subtle cues using one hand on the traditional "draped" rein. They aren't to "shout louder" at an unresponsive horse; though they do work in that capacity when misused as the curb action can and does multiply the force applied by your hands by a huge amount (longer the shanks the more this is).
If your horse won't respect "whoa" in SOME form of direct pressure bit (traditional snaffle, french link, etc) it is a training issue not a biting issue to be "solved" with a curb, because you aren't really solving the problem by upping the amount of force you can apply; you are masking it.
Anyway, I digress. If you are going to show western and your horse is over a certain age (I think it's 5, but check the aqha rule book to be sure) you have to show in a curb. There are good "transition" curbs and curbs that are good for the inexperienced to intermediate horse.
I, personally, would look for something with a low to medium port (this is for tongue relief, NOT to apply pressure to the palette) and at least independent side/shank movement if not a break in the mouthpiece to allow that to swivel independently as well.
The swiveling shanks will allow some amount of forgiveness when you find yourself needing to correct using two hands (I ONLY direct rein, not neck rein, in a curb bit and do so by moving ONE finger fractions of a cm in the direction I want to go. Thus, I'd contend there is a difference between direct reining with a curb and "shouting louder" correctional reining where you go back to the two handed and pull their head around or whatever).
A "jointed" port on the mouthpiece will make the bit more like the jointed mouth he is used to and make the side movement more independent (again, good when you need to correct, but still not the ideal bit for that situation. If you are correcting often you need to be in a direct pressure bit (no shanks)).
You can also get bits that do have jointed mouthpieces but aren't tom thumbs, and you might like one of those.
Examples:
"cutting/grazing" bit. No pivots, but a good "mild" curb for the advanced horse (probably wouldn't recommend in your situation, but it's one of my favorites):
Grazing Bit
Hinged port/swivel shanks otherwise similar to the grazing bit above:
Myler curb
Short shanked three piece mouth. The double joint and the extremely short shanks help address some of the issues with a tom thumb but give you a similar mouthpiece if you don't want to go to a port or your horse has a low palette. These will cause more tongue pressure which some horses don't like but others are just fine with:
Argentine Bit