There are several businesses that offer an ongoing teacher discount. (Most, if not all of these, include home school teachers.) I don't have a link for all of these, but check out the details for the ones I provide a link to. (I called the other locations or, in the case of Michael's, saw the Friday deal promoted in their ad last week.)
Most require you to show your school ID. If you don't have a homeschool ID, they usually allow you to show your Affidavit of Intent to Homeschool. (Check with your location to make sure they're participating and what their requirements are.)
Barnes & Noble: 20% off, including sales
Bookman's (in AZ): Register online, 20% off all purchase, doesn't have to be education related
Borders: 25% off list price on books, 10% off CDs & DVDs
Container Store: 15% off
Half Price Books: 10% off
Lakeshore Learning: 15% off (see list for items included)
JoAnn's: 15% off Sept. 1 – Aug. 31st, plus one 20% off coupon when you sign up,
sale items included, not with coupon
Michael's: Call to verify participation, since one store told me they give 10% off
non-sale items everyday.
Staples & Office Max also offer a rewards program where discounts are earned when you purchase a certain amount.