There are only two surfaces to clean on the prism. One is above the focusing screen the other is just in front of the eyepiece.
I have always used a bird's (goose,duck) feather cleaned in alcohol to remove the oils to clean surfaces like this and it doesn't leave lint. It also works well for the mirror.
Getting to the prism surfces is a little more complex though. The eyepiece surface you'll need to remove the top cover. The bottom suface you would either remove the focusing screen or the prism itself.
I'd suggest just cleaning the focusing screen with the aforementioned feather & let it go at that.
The foam can be had from Microtools or Jon Goodman who I believe is somewhere on APUG.