There's a great company named ULINE (http://www.uline.com) that has good pricing and availability for a lot of miscellaneous supplies we often need or use... for example...

Signage
Plastic corrugated pads (e.g. Coroplast)
Pre-made signage

Weatherproofing or Storage
Clear plastic tubes (ssrs? power connections?)
Zip-type reclosable bags
Poly tubing (string all your cables through one flexible UV-shielding poly tube)
PVC shrink film (wrap up a controller or wiring connection, heat-shrink this around it)
Jars (wide mouth and otherwise)
Shelf-bin organizers (storing parts, supplies, etc.)
Static-shielded bags (storing sensitive electronic components)
Silicone self-fusing tape - fantastic insulation for high-powered electrical cords

Other
Zip ties (all colors, sizes, even metal ties for super-strength)
Poly strapping (use for guy wires, hanging displays, etc. super strong, non-conductive)
Rubber/nitrile gloves (hand protection for home etching boards)
Copper tape (solderable, for Led Triks or other signs)
Other tapes - you name it, they've got it

Shipping supplies
Virtually everything under the sun