Savko - the best for hard to find plumbing supplies
McMaster-Carr - they have just about anything
US Plastics - everything made of plastics
Aquatic-Eco Systems - assortment of aquaculture materials
Ridout Plastics - tools for working with plastic
Modern Plastics - Acrylic tubing, sheets, etc.
Tapp Plastics - Acrylic tubing, sheets, etc.
Flex-PVC - Flexible PVC pipe,
fittings, metric to imperial adapters, etc.
O-rings Online - Everything having to do with o-rings
Uniseal Warehouse - Uniseals as very good prices
Delvie's Plastics, Inc. - Acrylic tubing, sheets, tools, etc.