I have found the Titleist Tour Distance balls only at Costco stores and nowhere else.

Besides Costco, I've seen them at various times at some other discount outlets.  Never see it with a full line of Titleist balls.

Wouldn't the Velocity be the replacement for the TD?

I had the same questions regarding this ball, since I am a huge Costco shopper. I primarily was a ProV1x player, but after a year off, I knew my wallet would not be happy teeing them up. So while in the process of getting my game back into shape, I thought the Tour Distance ball would be a good option. After several emails back and forth with PJ_Mattos of Acushnet, the closest approximation that was given to me is that they are most comparable to the Velocity. I have not played the Velocity, or the DT Solo, so I cannot make the comparison. However, in about 8 rounds, I found that if I compressed the ball well with my short irons and wedges, I could get the ball to stop, but anything longer than an 8 iron refused to hold the greens. The biggest positive in playing the Tour Distance, personally, was putting. It had amazing feel for a $15/dozen ball! It doesn't feel nearly as buttery as a proV, but it wasn't clicky like balls at comparable price points. I just purchased some NXT Tour S balls to try out, as I still don't believe I am consistent enough from the tee to play my proV's yet, but I am getting closer.