According to Jerking The Trigger, the infamous SB15 will soon be only available as a kit and no longer sold separately. The kit will include the SB15, buffer tube, buffer, recoil spring, and locking nut and the MSRP will about $239-$249. JTT did find out that the SB15 kit will be using the Phase 5 buffer tubes.
Since the SB15 retails for $139, you are looking at an extra $100 for the kit. However Phase 5 is selling the tube, spring, buffer, and the rest for only $79.
If you prefer to choose your own buffer tube, than you better get a SB15 before they are no longer sold separately.