Stems are individually fitted to each pipe, so it's generally not possible to get a generic replacement. There are pipe repair services which may be able to help. Occasionally sellers on Ebay offer groups of stems, but the chances of a match are not that great.
It's also possible that the meerschaums actually have amber stems, though the quality of the carving is not such that it seems likely.
If you want to tackle this yourself, get a good reamer such as the Pip net and some micromesh sanding pads, alcohol pipe sweetener and a lot of pipe cleaners.
The bear themed pipes are interesting and I can make out at least one GBD which might be worth some effort.