Since I'm guessing you don't want to spend a ton on a screen, you'll want to look at "Elite Screens" if you want to go the new route. Craigslist can be really hit or miss. Mostly miss from what I have seen.. Although it sounds like you did ok on the projector.
One site that has very good prices on screens with free shipping is:
www.projectorscreenstore.com
Your projector is a native 4:3 format. Remember screen size is measured diagonally, so to get 90" horizontal measurement in 4:3 you'll need at least a 110" diagonal screen. That might be a bit much for your projector at that size. 80-90" diagonal would likely be better to give you a bit more brightness.
If you have a Fry's electronics near you, they usually have good deals on inexpensive screens.
As far as painting a wall, there is screen "paint" available. Goo Systems is one brand:
Goo Systems, Screen Goo, Projector Screen Paint, Digital Signage | gooscreen.com
You can also use blackout cloth, which has a nice white surface.