You've had some good advise on this but I'd just like to add one little personal observation...
The times I have had a tight holster, I have had the best results by NOT wetting them or treating the leather with anything. Once you get that gun snapped in, you can rest assured the leather will eventually relax to the new fit. If you wet it, you might find the snap actually too loose down the road!
It's always best to be patient!