For example, coat polisher for cattle will often cost less than horse specific shine enhancers. The same goes for shampoos and other grooming products. Topical wound dressings and aspirin will also cost less in the cattle aisle. Talk to your vet to see what you can safely use on your horses if you have questions. Put items you have a ready supply of to use.
Any barn has an excess of hay twine.
You can fashion it into your own hay bags, lead ropes, cross ties, etc. Hay twine can be easily braided into tails to help them grow better and stay out of the mud. Use those empty feed sacks. Empty feed supplement buckets can often be used as water buckets, as storage for something else, or in the bathing stall. Or, get buckets that other people are throwing away.