Randy, I don't understand what Jet figures about accessories.
About the live centers, these are designed for turning metals. As such, they are subjected to a lot more stress than you can get from a pen mandrel, which is very light duty. It is just used to make sure that end of the mandrel runs true, you don't put a lot of pressure on it.
The Harbor Freight center, which is the same one as the LittleMachineShop cheaper one, has a single bearing, and it is NOT an axial thrust bearing. So for general metal use it is rated at 2500 RPM. I use one like it all the time, at about 3,000 RPM, and no problems yet.
The heavy duty model, same as the Grizzly, has TWO regular bearings in it. A center with a thrust bearing might cost up to several hundred dollars.