Wow. It's been so long, I don't recall. I think that I either ordered it from BikeBandit.com, or maybe eBay. Where-ever it is that you do go for yours, you might want to consider getting 3 of them - one that is the size you think you'll need, one that is a size larger, and one that is a size smaller.
Come to think of it, I might have bought a 'kit' of these that had a range of sizes in it.
BTW, the bike is still running great. The mods were well worth the effort.