any urathane reducer as long as its med or fast. Most use Med because its lower content of Toluene.
There are a couple specific brands that most use but that's not for me to say in a public forum. The best thing to do is ensure you add plenty of whatever oil you decide to use with it. 8-14 ounces is common per gallon. Also any oily chemical that helps with bite will need to be added, this ratio for you will be trial and error. once you have a solid mixture, stick with that to keep your program as consistant as possible.
We sell everything to make your inside, from 4 different types of reducers, to 2 different grades of Toluene.