It won’t let me save my edits, so:
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Woodcraft of Boston, Woburn, 185 New Boston St, Woburn, MA 01801, (781) 935-6414 A bit pricey and selection is iffy, but they have some thin boards, as well as pen blanks (wood & acrylic), plus tons of stains and other finishes.
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Tandy Leather, 73 Princeton St. #10, North Chelmsford, MA 01863, 978-275-0596
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Lowell Makes: http://lowellmakes.com/ - makerspace in Lowell. Has a GF and a Full Spectrum. Members get discounts to some local places, like Tandy and Woodcraft