Hi Shawn
My experience of machining them is that plastics normally don't come in 8 x 4ft sheets - the base size is much larger (often 3 metres x 1.5 or 2 metres) - although most phenolic stockists I've dealt with will saw to size. I'm not in the USA, but I'd suggest looking for "Engineering Plastics" or the like in Yellow Pages. Over here we have a manufactured brand called
Tufnol and they do have distributors in the USA, including
Complex Plastics Inc, although a quick email to Tufnol may furnish you with a nearer distributor.
One thing I will say is that you must have deep pockets because an 8 x 4ft sheet of 3/4in thick Tufnol is not going to be cheap (hundreds of dollars). It is more normal to use Tufnol to face another cheaper material, say a 3 or 6mm (1/8 or 1/4in) thick skin on 19mm (3/4in) plywood
Good luck with your searches
Regards
Phil