For parts and accessories find the link on the Guillemot site to the SEE Kayak Directory. Go to Businesses, then Plans and Kits. Many of the kit manufacturers sell materials and accessories. CLC has as good a selection as most.

As far as choosing 'glass weights, you should probably back up a step and mention what construction method you are using and what sort of use you expect the boat to get, i.e., the Colorado River at flood stage or the backyard frog pond.