There's not a homebrew supply anywhere near where I live. Is it ok to use muslin bags from a previous batch?
As long as they're serviceable. If they're used only in the boil kettle just rinse well after you use them. If they are being used in a secondary wash/rinse well and sanitize before using.
I actually throw mine in the washer, I only use a mix of borax and washing soda, no perfumes or dyes.