I found mine used at a gunshow and got a great deal on a relief carved set that were .20" thick. Sorry to say I would have no idea where to acquire more just like that.
However you may be able to find some .25" thick (and if you are brave you can have them thinned) from a gentleman in Indiana. He sells Elephant Ivory stocks (among a plethora of other materials) and you can email him at [email protected] The company is Colinscraft Grips. Phone 812-988-0418