I think the most common mistake when trying to wet mold holsters is not waiting for the leather to dry enough. If you work the leather when it is too wet the leather will not hold that sharp definition you are looking for.
I agree with Shooter, you can get good results with Tandy leather provided you are using veg tanned sides, shoulders or backs, not bellies.
I personally prefer Herman Oak or Wicket and Craig for commercial use. For hobby stuff, you just can't beat the convenience of Tandy.
Best Luck,
Jeff