Here are a few places I've used for various DIY projects over the years:
www.boltdepot.com
flexpvc.com
www.kjmagnetics.com
I've never ordered anything online from these next two but I live close by so those are my sources of materials, usually from the scrap bins:
www.eplastics.com
www.industrialmetalsupply.com
Anyone have any more to add?
Bolt Depot - Nuts and Bolts, Screws and Fasteners online


Flexible PVC Pipes, Hose, Tubing & Fittings at Great Prices With FREE Technical Support- #1 FlexPVC® Official Site
Don't buy from Amazon, they do not sell FlexPVC, they sell imitators. See our selection of flexible PVC pipes, fittings, clamps, corners, hoses, cement, primer, tools and much more today! At FlexPVC, 90% of our PVC pipe and plastic pipe fittings are made in the USA. We offer wholesale discounts...

K&J Magnetics - Strong Neodymium Magnets, Rare Earth Magnets
K&J Magnetics - Incredibly strong neodymium magnets at affordable prices. Large variety of stock rare earth magnets available.

I've never ordered anything online from these next two but I live close by so those are my sources of materials, usually from the scrap bins:
Plastic Sheets, Rod, Tube & Fabrication Services Since 1914 | ePlastics®
ePlastics is a distributor of plastic sheets, rod, tubing and fabrication trusted for over 100 years. We carry a large stock of plexiglass, acrylic, polycarbonate, fiberglass, hdpe and other speciality plastics. Based in San Diego we ship worldwide.
Industrial Metal Supply | Metal Supplier in CA, AZ, and NV
Anyone have any more to add?