Watonga area is about as close to OKC that I can remember seeing them. I'm sure there's some closer, you'll just have to put in some drive time and check. Here's a couple of recipes that I've used, but there's a bunch of them on the web.
https://www.youtube.com/watch?v=f_w_ajq8KIQ
Also tried this one.
http://www.cooks.com/recipe/m76y11tu...lum-jelly.html
Attention for anyone having trouble setting: Wild plums are low acid, so you will need to add 1/4 cup of lemon juice (or equivalent acid) for the jam to set. I'm just finishing up a 35 (8 oz.) jar batch of wild plum (prunus americana) jam and the first 10 jars are syrup but the rest, with 1/4 lemon juice added turned out phenomenal. My recipe is below:
4 cups wild plums
4.5 cups sugar
1 package sure-jell pectin
1/2 cup water (from the initial plum cooking)
1/4 cup lemon juice
Put the wild plums into a sauce pan and fill with water until covered. Cook until most or all of the plums have split open. Save 1/2 cup of this plum water. Put the plums through a food mill (I used the Oxo brand food mill). Place plum puree into saucepan. Add pectin, 1/2 cup plum water, and 1/4 cup lemon juice. Stir constantly. Bring to rolling boil. Add 4.5 cups of sugar. Stir constantly. Bring to rolling boil for 1 minute. Remove from heat and place immediately in jars. Use your own canning process. Enjoy.