I get my latex from
The Monster Makers. I've used their RD-407 latex for casting masks and for building up the layers on my Mr. Incredible costume. They also have a lot of related supplies there, such as clay and tools for sculpting, Ultracal 30 for casting, and paints that work very well with their mask latex. I haven't shopped around to see if there are cheaper places to buy it. If you lived in or near Hollywood, I'm sure that ptgreek (a mod here and a special effects guru) could point out all the cool places.
If you're going to be making your own biohelmet, doing the molding and casting for that, then the best supplier I can recommend is
Smooth-On, which has all sorts of supplies for making silicone molds (both brush-on and pourable silicone) and for making casts in a variety of plastics. You can usually find their stuff at art stores like
Blick Art or another art store in your area. They have trial sizes and starter kits of their molding and casting materials available, so it's a great way to learn. Again, if you're into serious Hollywood special effects, someone like ptgreek could tell you where to find the fancier stuff.