Alex's idea is nice if you're handy with needle and thread (and/or sewing machine ). I'm not, and following Drax and other's wisdom I got mine from Arkady and couldn't be happier -- she's a great source for any of the soft parts for Fett.
One thing to consider if you can't sew is asking a friend who can sew for help, or for a work trade for a skill you do have. I know a few people who sew for fun and for profit, and some of them would find helping for a Boba Fett costume intriguing and fun. That said buying them is easy as a few clicks once you know who to get them from.
I'm not sure where you're located, but I have a set of MoW spats that I don't need. I went ESB and dyed them blue to use until I got the proper ties. They'd need to be re-dyed but I'd sell them cheap. PM if interested. I'm in Canada.