Another person who orders virtually everything from amazon, rainbow resource, and discount school supply here!
I use amazon primarily for books to read (story books, easy reader, etc.) as well as general homeschooling books for myself. It's also great for random little things throughout the year, like pencil grips, and for things like cap erasers that I just don't feel like driving 45 minutes away to an office supply store to pick up. I have a prime membership so everything is free and quick shipping.
I use rainbow resource for all kinds of things but I can't stand paying for shipping so I usually only get up to the $150 free shipping minimum once a year. They have wonderful customer service and pack things very well, the only time I had even the slightest problem they did everything possible to fix it.
I use discount school supply for craft supplies. I get fine art materials locally or through rainbow resource but discount school supply is great for crafty stuff. I've gotten amazing deals on beads, popsicle sticks (a box of 1500!), tempera paint, felt, foam sheets, and so on. Decent quality and inexpensive enough for the kids to make crafts every day.
The only other significant orders I place each year are through math u see (their books have to be purchased through them) and through seton (a Catholic curriculum company).