I have a friend that said he would build me a bow with a phenolic riser if I could find some. Where can I find some larger blocks for a solid riser? Needs to be at least 18" long by 1 1/2" wide and 2 1/2" thick. Is there any large companies that sell it or maybe an individual? Would like to have black but don't know if that matters.

Here is a site. You could glue correct size pieces together.  web page

$$$$

Click on search up top, put in phenolic riser and check out the sources listed......  

For what it's worth, G10 is some pretty cool stuff, instead of a cotton/linen weave like phenolic, it's a glass weave in epoxy. It's commonly used in knife handles, I've used it on tips and overlays, it's virtually bomb proof and opens up new color options.

I found a few options so far. Will take them to my friend and see what he says. Looks like it may be a little pricy but have to pay to get something nice.