The 36ø rule is a good general guide for 2.35:1 and 1.78:1. At a pinch you could use 40ø which will give you a bigger screen size.
There are many brands of screen available to suit all budgets, but I would advise that you buy the best screen you can in proportion to your projector spend. You should really consider the combination as a single purchase and take into account factors such as ambient light levels. This will help you define your fabric requirements.
If you would like a price for a Screen Research screen, then please feel free to contact our UK distributor, Pulse Marketing (www.pulsemarketing.org) or email me on gary.mardell@screenresearch.com. But be aware, that we will not be your cheapest option.
hth
Gary
P.S. Don't think I broke any house rules there... I hope.