Western people need some bit advise?

Well, the fact that you say "sadly, I have to show her in a shank bit" indicates to me that you are not clear on why a shank bit is used, and how a horse is prepared for finishing into a curb. I think you may be rushing into something neither you nor your horse have been adequately prepared for.

The purpose of transitioning a snaffle trained horse into a curb is to lighten your aids and establish refinement that the snaffle doesn't accommodate. When done correctly, there is nothing sad about it. In fact, the horse is given the opportunity to avoid all forces placed on the mouth tissues by learning to respond quickly to vibrations coming in through the shanks before any force is applied.

It is a long process to fully snaffle train a horse before beginning the process of transitioning into a leverage bit. the horse should already be fully responding to your body aids and to the neck rein. You cannot use direct rein for lateral aids in a curb bit. All lateral work is done through your body aids and the neck rein. The horse should be able to be rated, halted, pivoted, moved laterally, and backed with body aids and neck reining before the curb bit transition is attempted.

In the snaffle, the horse is trained to reach for the bit to seek contact. The horse is driven forward and reaches into the bit to come into contact. This is not the case with a leverage bit. The curb bit backs the horse off of contact with the bit, which is the opposite of the snaffle function.

It is only engaged to lower and flex the poll, telescope the neck, and to refine suspension or collection. Ideally, feather light signals from your fingers to the reins create the vibrations delivered through the shanks so that the horse is responding before the shanks are even rotated backward.

If all of this is in place, then I advise using a mullen mouth Pelham bit with both snaffle and curb reins to start the transition. The snaffle reins can be used for all control and direct signal while you adapt the horse to the feel of leverage and give him time to learn to respond to vibrations from the shanks before it is engaged. The curb reins should be loose, and used for neck reining and very light signals through the reins. You will only need to actually engage the leverage a few times before the horse learns to avoid it by quickly responding, provided you do this right, and release your signal the second the horse gives you a response.

You can use an English mullen mouth Pelham for this, or there are cowboy Pelhams with a medium port which are fine as well. I actually like some of them better because they have curved shanks which more closely resemble the finishing bits I recommend.

Please don't use any jointed mouth curb bit, Tom Thumb or otherwise. This design does not function correctly as a leverage device and I never recommend combining shanks with any jointed mouth bit. Not only is it impossible to deliver clear and correct signal to the mouth, ut it is potentially and unjustifiably extremely harsh.

I don't show and after trying many bits including a western curb I finally settled on the Dr. Cook's bitless bridle. It is the only pain free type of bridle out there but of course you can't use one to show. I only answered because of the Tom Thumb refferals. They used to think the Tom Thumb bit was a mild western training bit but after studying it they found that it had too many moving parts and was painful and confusing to the horse. It is no longer recommended. You can look it up on line and read all about it.

Gallop's advice is good. Make sure you know what you're doing and how to use the bit and your horse is ready for it before just putting a shanked bit in her mouth.

I would recommend a bit with short shanks and a mild port, even one with a roller. Something like this

http://teskeys.com/francois-gauthier-low-port-hing...

Or even something like this that doesn't have a port but has a roller

http://teskeys.com/bob-avila-straight-roller-bit.h...

You could get something with a low port without a roller too. I just like rollers personally.

one of My best friends rides western these are the bits she uses and the horses she uses them on

10 year old, QH mare, pleasure and reigning

http://www.statelinetack.com/item/ss-copper-roller...

5 year old, GREEN Paint gelding, gaming and endurance

http://www.statelinetack.com/item/ss-tom-thumb-wes...

7 year old, Appy mare, VERY sensitive, trail, barrel, poles

http://www.statelinetack.com/item/reinsman-flat-le...

all the bits are very nice and the horses work perfectly! just be extra careful you dont yank on the mouth if you choose the 2nd one as it is a stronger bit, my friend has very soft hands and mostly controls the horses with her legs anyway so she can use any bit gently. My personal fave of the 2 non hackamore bits is the D ring!

hope this helps!

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You don't have to use a shank bit against your will.

A tom thumb would work well. My horse hates a tom-thumb but that's the best shank bit out there.

http://www.valhallafarm.net/TomThumbBit.jpg

i would use a simple tomb thumb snaffle i use them there gentle or a roller bit