Look at the big picture. As a homeschooling family, materials for learning are all around you. Why buy a package of math manipulatives when you've got measuring cups in the kitchen or 12-piece socket sets in the garage?
Plan before you buy. Review your curricula for the coming year, do some comparison shopping for the supplies you'll need, set your budget and stick to it.
Involve your child or children in the planning and purchasing of supplies. Homeschooling is all about giving children the opportunity to learn in real contexts for real purposes. Think of the logical reasoning and math skills involved!
Buy incrementally. Especially if you are new to homeschooling, it will be hard to judge in August how much graph paper your 12-year-old will really need over the next nine months. The small savings you might realize for buying in bulk will mean little if half of your supply is never used.
Think collaboratively for large ticket items such as copiers, musical instruments and telescopes. You probably don't need one of each in your house, you just need access to them. Get together with other homeschoolers and share resources.