Any square hood in the Rollei/Yashica Bayonet I size will fit. I think $100 is a really good price for a mechanically sound 635, especially if it includes the 35mm kit, which is usually missing.
Here's a sample listing for a suitable hood, priced too high in my opinion:
http://www.camera-depot.com/TLR.htm
I agree with ColdEye about a hood being necessary. Some patient searching on eBay will probably turn up one at a better price.
I had a 635, fine camera, wish I still had it. As long as they work right, I wouldn't care about those black aperture/speed wheels. But that's just me.