Scott has given you some great choices, may I add a few of my favorites as well: black cherry,southern red cedar,black walnut,and mahogany. No matter what wood you choose it should be a tight grained wood which is hard or dense. The hardness and density of the wood along with the surface upon which the striker is worked will determine whether you are going to get a high or low pitch. Remember that not all slate and not all wood is created equal. Match your striker to your surface the best you can. A striker which sounds good on one pot may not sound as good on another.
Hope I didn't ramble on to much!
Dixie Belle Handcrafted Turkey Calls