Assuming you are looking for film (why else would you be here!), I don't think it matters which major brand or model. The most important things are they are capable of fully manual operation, are in nice (as in working!) condition, and they have a single focal length lens of about 50mm. The Pentax k1000, Nikon FM, Canon AE1, Olympus OM2 are all fine....just avoid anything requiring an old mercury battery like an OM1. I love the OM1, but it isn't worth the battery hassle starting out. I personally think that manual focus (all camera mentioned above) is an important feature. When you take the picture you will know exactly where it is focussed. As far as where to buy, I'd either buy it in the classifieds here (good, honest folks) or from KEH.

Mark