M&F brushes are hard to come by. If you're lucky, you can contact Lee Sabini and have him make you one; getting in contact with him is very hit and miss. You can check the GFD and the Marketplace, they appear there on occasion (read that as rarely), or post a WTB in the GFD. You might get lucky.
He still makes them, but it seems that only the high rollers like Ben74 and Chiefbroom can get them. To be honest, the knots coming out of Lee as of late have left a lot to be desired. Rudy Vey has some SPECTACULAR Shavemac 2-band SiLVERTiP knots that blow the M&F knots away. Super soft and dense like Simpson's Manchurian (that's a great knot).
My latest acquisition from Lee (3 months waiting) is an Alibaba 1 and even though it is a nice knot, I reach for my Rudy Vey brushes more...Add to that for a typical M&F you could almost get TWO Rudy Vey's with Shavemac and there you go.
If you really want to fight the Lee Sabini battle then the only way to get them is by sending (spamming) his inbox with emails and hope he is in the mood to answer you. Fickle fellow, but he makes knots for the whole world, so I suppose he is a busy guy...
If you email Florian Arnold (get the email address from their site) he is pretty decent about replying. These days he is only making customs. The wait can also be a few months but they are wonderful brushes.
The recommendation for a Shavemac 2 Band knot through Rudy Vey or even buying from Shavemac directly is also a good one. Those are excellent knots.