The "rubber" stuff I'm talking about is like the stuff that is put on the bottom of infant and toddler socks for grips. There's usually a pattern (like paw prints) or the company slogan on them. I think I've seen it at Michael's and I'm sure any craft place would sell it. In fact I think companies like Barbie and Crayola also have kits where you can put your own patterns on clothing.
Doing the felting in the sink sounds like a good idea to me. That way I won't have to wait until I move into the house to get at my washing machine
It also seems like I would have more control over the agitating process, plus I would be able to see it happen. Maybe I'll try those slippers next after I'm finished my hubby's hat.