I have gotten things used at our co-op, but that's over for the summer now. Also it's only open to co-op members, so there's that.
I have also gotten used curriculum from ebay, and the homeschooling area of diaperswappers.com's forums.
If you do swagbucks, you can earn Amazon gift cards, which can decrease the cost of the items you want to buy.
I have an Amazon credit card which got me $30 off the first time I used it. You could do that and then never use it again. I do use mine (pay it off each month so no interest) because the $ I spend rack up into $ I can spend on Amazon. It's also in many ways easier to just use the Amazon credit card for gas, groceries, etc, then have ONE bill to pay per month, than keeping track of dozens of debit purchases coming out of my checking account.