I'm using Poison Ivy, pretty similar to Velocity and I had the same question when I switched from an 8mm Distel Split-tail to a Blake's... Where do I get an 11mm split tail?
First I tried a regular 1/2" 12 strand split tail and didn't like the bulky knot.
So I cut a 5' section off my rope and used it it, works fine but i'd just as soon have a different color for my split tail and I didn't want to cut another 5' off or use with a double crotch... which was why I went to the Blake's in the first place.
I thought about buying 150' of velocity as a second rope and cutting 5' off of it... I still might do that.
But what I found is that a 1/2" Tenex split tail from Sherril's works like a champ. I really like it... broke in fast and it's soft and controllable, better than the 5' of PI I cut. The only drawback is that it supposedly wears pretty fast... time will tell, I've only used it a few times so far.
When you descend on a Blake's, do it slow. The bottom wraps takes a lot of wear when you descend.