I got them done at Hackvana (hackvana.com, or #hackvana on irc.freenode.net); similar prices to Seeed, and excellent customer service.

I'm not sure about the maximum speed. The flipflop, for instance, has a delay of 14ns; if we take that as average, each slice has to go through 5 parts (input mux, LUT, flipflop, async select mux, output enable) for a total delay of 70ns, so in theory one slice could do about 14MHz. Since it's a ripple counter, I guess we should divide that by the number of slices, so 4MHz seems like a reasonable upper bound.