You can buy epoxy tackifier, I have a bottle of EMP around someplace that I used with prepreg years ago--it's probably a solid lump wherever it is. I remember it being stupidly expensive, but everything involved with prepreg is. Most folks just use a light coat of 3M cement--cheap and it works. Just don't get too carried away or it might interfere with wet out and/or the layer to layer incorporation. Are you bagging dry and infusing? If you table wet the layers you don't need a tackifier.