You are buying the muslin for a specific purpose and will be using it, not just buying because you want to so I think it is an ok purchase. But if you really want a justification here you go: Yes, I could indeed tie-dye my own muslin (which I would have to buy for a higher price) but it would require a significant investment in a large commercial scale stainless steel pot not to mention the tea or other supplies, so I am saving the budget even if I'm purchasing fabric.
Grats to you for getting your projects done and out to people.