.300 blackout uses the same buffer sizes as its 5.56x45mm counterparts. You will not likely find buffers and receiver extensions listed specifically for. 300 BO. It is important that the buffer size is correct for the receiver extension in which it is installed.
I assume that you are looking for a buffer/receiver extension combination that is shorter than the typical carbine length. If so, then look at the 5.56 options. You may wish to vary the weight of the buffer for best functionality, depending on your load, and exact configuration of your upper assembly.