Looks like it’s the exterior left outer boot (#9 in the diagram)...so it is replaceable. You can get the part nbr at the link below and from there you can google the 11 digit BMW part nbr and perhaps find vendors that sell the part. If it's not a well stocked item, you'll need to call your local BMW dealership's Parts Dept...give them the number...and they can tell you how much it cost from them. Good luck.
And it looks like there's instructions on the TIS website...there its called the left or right "gaiter":