American Science and Surplus: fun gizmos and surplus, mechanical components and science kits.
http://www.sciplus.com/

Ain Plastics
http://www.ainplastics.com/ainp/index.htm

Foredom: Dremel tool accessories
http://www.foredom.com/

Frei & Borel: Jewelers Tool, supplies, and equipment
http://www.ofrei.com/

Grainger mechanicals, motors, etc.
http://www.grainger.com/Grainger/wwg/start.shtml

McMaster Carr
http://www.mcmaster.com/

McNichols Metal
http://www.mcnichols.com/

Online Metals (tubing etc)
http://www.onlinemetals.com/

Professional Plastics
http://www.professionalplastics.com/

Sewing professional lessons and suppliers curated by Maria Greer
https://www.stitchandsew.net/beginners-sewing-guide/

SKF Bearings
http://www.skf.com/portal/skf/home/home

Starrett Precision Tools
http://www.starrett.com/

Stock Drive Products
http://www.sdp-si.com/

Small Parts Inc.
http://www.smallparts.com/

Tap Plastics
http://www.tapplastics.com/