In case it's helpful for bigger orders, I've found two main sources for these long-square-tailed-female-headers.
The first is Samtec:
They're pretty pricey, but they do deliver very quickly.
The second is Don Connex (whose UK distributor is Toby
- they've already been mentioned)
I'd happily recommend Toby by the way. Friendly and efficient.
There's a third source, which is 4uconn, but I don't think their tails are quite square (but I'm not sure that matters). Their part numbers are 18688 and 18689.
The samtec parts above are the gold-flash version. Below is a picture of the matte-tin finish version.
[edit: And, as is often the case, I've completely misunderstood the question! - My waffling was for the long-tailed headers that are suitable for stacking shields - the OP just wanted standard female headers... Oh well, someone might find the info useful <sigh>]