Most original Glastron stuff isn't available anymore, but you can get creative ...
Original tops would be rotted and shrunk, but you might be able to find something on eBay, could take some time though.
Get some measurements of frame, width/height on your boat, pick the style you like, and do a Goolge search for "Boat tops / Biminis".
OR ...
Bring to canvas shop or Upholstery shop and get a estimate.
Look here for stuff ...
Vendor and Repair Resources
http://forum.cgoamn.com/index.php?board=7.0