I thought I might as well make myself useful here and list some local and online sources for good DIY supplies. Please post other recommendations here and I'll consolidate them all in this first post for future reference.
J. Freeman, Inc. Acrylic sheet, scraps, glue and various acrylic-working supplies
65 Tenean Street
Dorchester, MA 02122
(617) 282-1150
or (800) 841-9442
www.jfreeman.com
Altec Plastics Inc acrylic stock, scraps, glue and applicators, etc
116 B St Boston,MA 02127
617-269-1400
Tap Plastics Acrylic supplies online, will cut acrylic to size for you
www.tapplastics.com
Rockler Woodworking cabinetry hardware, router bits, finishing supplies, etc
2154 Massachusetts Ave, Cambridge, MA 02140
(617) 497-1136
373 South Broadway, Salem, NH 03079
(603) 898-5941
www.rockler.com
Woodworker's Supply lots more stuff than rockler carries, and some of it higher quality (but only online)
www.woodworker.com
Woodcraft high quality hardware, hand tools, and finishing supplies, local and online
313 Montvale Ave, Woburn, 01801 - (781) 935-6414
www.woodcraft.com
CabinetDoorsandHardware Very inexpensive cabinet doors in all sizes. Wide variety. Looks like good quality. See pics in this thread.
www.cabinetdoorsandhardware.com/
F.W. Webb Plumbing Supply commercial plumbing supply warehouses
Locations all over the area
www.fwwebb.com
Savko All manner of tank plumbing supplies - true unions, bulkheads, John Guest fittings, etc etc
www.savko.com
US Plastic Acrylic stock, eggcrate, various bottles and tubs, PVC plumbing parts - basically all things plastic
www.usplastic.com
McMaster-Carr Industrial Supply catalog - everything from acrylic stock, electrical components, and plumbing components, to big rubber gloves
www.mcmaster.com
Grainger Industrial Supply Warehouses and online (sometimes they'll sell retail - sometimes they won't)
www.Grainger.com
Boulter Plywood Wide selection of top-quality plywood, and a fair amount of hardwood lumber as well
24 Broadway
Somerville, MA 02145 (617) 666-1340*
www.boulterplywood.com
Aquatic ecosystems Flexible pvc,Venturis, Beckett, Plastic Needle Valve, 1/4"
www.aquaticeco.com
E-Conolight.com cheap ballasts and electrical supplies
www.e-conolight.com