I will second NBBC13's recommendation. An excellent extremely cheap substitute are the nylon paint strainer bags from Home Depot, Lowes, etc. They sell them in 1,3, and 5-gallon sizes and only cost a couple of bucks for a pack of them.
When I was still using the Bayou Classic pot for brewing, I found the 5 gallon bag would fit around the entire lip of the kettle, and I would secure it with a couple of binder clips.
Nowadays I use a much larger kettle so those paint strainer bags won't fit. I have since switched to using the fine mesh nylon filter bags that are commonly used for aquariums and the like, and hang them from my mash paddle (which I place over the pot) during the boil for my hops additions.
For grains, I custom-made a BIAB bag for my kettle using $12 worth of materials I picked up at a fabric shop.