I think the availabilty of gallium arsenide has been looked at in detail by this forum, for example see a post from oogee (there are likely many more recent posts as well):

http://agoracom.com/ir/POETTechnologies/forums/discussion/topics/581162-enough-gallium-and-arsenic-available/messages/1823234

I am not sure about the cost of the raw materials, but from what I recall the whole manufacturing process will be significantly cheaper than that with the existing silicon process, so likely a non-issue. Also, when there is a greater demand for gallium arsenide, I suspect it will come a lot cheaper. Someone might even open up a gallium mine if warranted, currently there would not be such a need I suspect. As far as arsenic goes, as an Environmetal Chemist, I can say that there is too much arsenic around in the world, although I do not know how that relates to required quantities, but I suspect no issues at all.

Cheers,

Quan