Just for testing out, have you considered something like this? Western NP Sweet Iron Loose Cheek Curb Bit - Statelinetack.com
Don't let the price scare you off. I ordered one several years ago just to see if it would be worth having it they are nice bits for the price. Of course, nothing fancy, but they're functional and good quality. That at least gives you something to try that doesn't cost an arm and a leg.
I know that a lot of people (including professionals) use those broken mouth curbs, but I just really don't like the feel of them. I get a much better feel from something like what I posted above or something like this, which is very similar but it has the myler type mouth. I have one of these and my horses really like it. Saddles Tack Horse Supplies - ChickSaddlery.com Francois Gauthier Antique Hinged Futurity Bit
I've ridden lots of horses and almost every horse I ride goes into one of these types when progressing up from the snaffle. So long as I did my early training right, I've never had a problem with a horse not liking at least one of them.