I don't have any specimens on hand but I used the rough bark of a Flowering Dogwood to prepare moist culture chambers.  Other substrates may be just as productive.  Incubation time was about 21 days for P. roseum.  Soak the bark or other substrate for about 2 days in distilled water, then transfer to petri dish or other suitable container with damp filter paper or paper napkin in place and date with China marker. Nutrient preparations may also be used and experimented with. Store cultures at room temperature in subdued light or light tight container.  A small styrofoam cooler and plastic lab or borosilicate petri dishes can be used. Monitor daily, a good Hastings hand lens or dissecting microscope is handy for observations.  Identification to species may require a good high powered transmitted light microscope for spore observation of fruit bodies.

Not sure about roseum but BasseFosseProduction sells Luna (didymium aquatile) on Etsy at https://www.etsy.com/uk/listing/1211525981/set-of-3-white-blob-scle... I got mine from there. They have a few differant clones of Physarum polycephalum and I have seen roseum on there before (I've been looking for one myself for a while but unfortunately missed my chance).