The answer to your question isn't straight forward, by in large there are no ready made replacement stems for pipes unless it's a cob, Falcon, or some meerschaums. Most stems are made to fit each pipe individually, replacement stems are likewise custom fit to each pipe. Unless you have access to a lathe or tenon cutting tool your best bet is to send the pipe you wish to have another stem made for to a pipe repairman.
Pre-molded stems can be bought from various places like Vermont Freehand but these stems will need to have the tenon sized and the exterior cut to match the shank/pipe.
Some pictures of the pipes you'd like to change the stems on might help.