Hi Steve,
I used Maritime Plastics in Annapolis (910-J Bestgate Rd., (410)263-4424 (just behind Annapolis Mall off US-50) for cast acrylic for 2 big overhead hatches and 4 opening ports. They are easy to get to, experienced and easy to work with. I had the tinted acrylic cut for four small Gioit ports (4.5" x 9.5" x 0.25 thick) for $91.23 plus tax.
They will charge $75 to layout EACH job and then the cost of the material. Their cost of material is comparable to other plastic shops, so if you have many pieces for the the same job it may work out. Just bring them a port lens to get an estimate. You can also call them and get a rough estimate. They will also drill any holes and finish the edges which it looks like you need ( I didn't) which might cost additional.
For single pieces like a hatch there may be cheaper ways to go as the $75 layout fee doesn't get spread out over multiple pieces. My 20 x 20 x 0.5 tinted hatch was $130 ($75 + 55) and the 25 x 25 x 0.5 was $170 ($75 + 90). This was a little high due to the layout fee but I wanted to get the hatches done without finding another shop.
FWIW, any local plastics retailer will be much less expensive than getting the acrylic from the original mfg. The acrylic from Atkins Hoyle was $400 EACH! And it does not come drilled for latches if you have them. Rigrite now handles the parts for Gioit ports so you don't even want to know what that "custom order" would cost.
The only other plastics place I've heard of near us is Read Plastics in Rockville (301)881-7900. Found them after I did my hatches. I will get an estimate from them for my companionway acrylic project.
Mark