It looks like the only BiSS modules are 3rd party CRIO units. The cheapest option would be a dumb incremental encoder and a NI USB module that includes counters. Not all of them do, read the spec sheets carefully and then speak to your local NI sales engineer. It is their job to advise on this kind of stuff.
You could bit bang BiSS: the protocol is open.
There are iC-MB3 and iC-MB4 usable with SPI.
There is a BiSS master IP in VHDL available with a BiSS license (free of charge).
It debends on the requirements of Your BiSS application...