I like to make a list of what we'll need for the upcoming year. I plan ahead and usually start this list in late spring. Items go in a few categories - curriculum to buy, items to supplement curriculum, and stuff to buy at back to school sales.
Curriculum:I like to track what the price for each book brand new is at Rainbow Resource. That's the price I'll use for comparison when I shop everywhere else. I check what editions are available, and ensure I'm purchasing the correct one for our needs. Then I start watching websites. I buy most of our math and some language arts curriculum used from Ebay. I have purchased from Homeschool Classifieds and the Facebook group Homeschool Curriculum Swap, but I prefer a website with a stricter review process. I've found some great and honest deals on the Facebook group, and I've had some scams and inaccurate descriptions on classified groups. I'd rather be absolutely positive about what I'm buying, so Ebay is my favorite. I also prefer to resell there.
When I do have to buy something new, Rainbow Resource usually has the best prices. Plus, they have SO MANY other items available! We've done plenty of birthday and Christmas shopping from that site.
Supplemental Items:Most of the supplemental items we need are for The Family School, our curriculum that covers history, literature, science, geography, music, and art. I go through the curriculum before school starts and make a list of what is needed in each lesson (these lists are available in the Worldwide Facebook group). Then I can plan throughout the year to buy supplies ahead of time. I have found it less stressful to buy everything in the summer. I create an Amazon Wish List with the supplies I'll need, which forms another list of average prices. I check if Rainbow Resource has anything needed, as well as local stores.
We often need books for The Family School. I make a list of the books we'll need throughout the year and see if they are available for our Kindles, what the cost is, and if the library has a digital or paper copy to borrow. For literature books and other books we need for more than 2-3 lessons, I look for them used. Again, Amazon is helpful. I have used Abe Books as well, and though their descriptions are identical to what is on Amazon, I have found a range of variance in the quality descriptions of their books. There is no rating system on Abe Books which frustrates me, especially the books that are marked good and are falling apart and stained when they arrive. Amazon has great customer service if something is mislabeled.
Other Items:This would be stuff like binders, pencils, erasers, Sharpies, paper, glue, paint, etc. A few years ago I printed a list that shows average item prices as well as back to school sale prices, and super inexpensive "stock up" prices. I carry this in my purse and patiently check ads and stores at back to school time. We have so many twenty-five cent boxes of crayons that we could supply the block, and glue sticks that we should be set for the year!
My kids found which mechanical pencils and pens they love so we are picky about those, but we still watch for sales. We also have specific binders we like to use and usually get them on Amazon. Costco has binder packs for decent prices too. If you subscribe to Staples emails you'll frequently see rewards offers on a ream or case of paper. You can get a ream of paper for one cent with rewards or rebates!