Try Credo Interactive. It sells mo-cap bundles specifically for Poser (both human figures and animals), at very reasonable prices.

DAZ 3D has or brokers a few bundles, also.

wolf359, cmdctrlesc, davidm, and pdblake all have some stuff here in the Marketplace.

If you have money to burn, try Turbo Squid. A single file can easily go for $25 or more there.

You can download free BVH files off the Internet, but they'll often need work after importing into Poser.
Link #1 (30MB download, lots of files)
Link #2 (a few dozen more)