The Promaster uses a Humphrey HEA10F5-GL-101W solenoid. If your looking for the stock, you can find it
Here
You might also be able to contact Vaporworks at
www.paintballtrader.com and see if they can get you a better deal on one.
Honestly, it might be a good Idea if you have the money, to just buy a new Promaster. Seeing as you can pick one up for around $100, you could swap the solenoid's and then have all the spare parts you need in case something else goes wrong.