You can buy live sphagnum on ebay.
But it is not suitable for covering pots.
What you need is terrarium moss and that is also available
on ebay.
This type of moss needs to be kept cool, partly shaded and watered
with distilled, rain, or R/O water in order to stay alive.
Even with this care it can die off because of variations in the optimal
temperature shifts between night and day as warmer nights can be
exhausting for the mosses.
There are other toppings for planted pots that are not so demanding of care and are more affordable to replace or renew. Check the link below.

Here is a link that might be useful: Top dressings

It's kind of cheating but I think some of the stuff they sell in pet stores is dyed green. It's probably preserved or something but it's kind of a safer bet than trying to grow moss. That being said nice healthy, live moss is much better if it actually stays that way.

Sometimes when you purchase a bale of long fiber sphagnum moss (dried) sold for orchids. It's has happened that sphagnum moss spores will germinate in time, but those are rare occurrence.

but if you want moss for your carnivorous plants, do not use moss sold at stores. They will be toxic for your plants, & often they are dyed green but the dye will kill your plants. Use only pure long fiber sphagnum moss or sphagnum peat moss, make sure that it doesn't have additives like wetting agent or plant food.

It might depend on the plant and I'm afraid to use it but preserved sheet moss is used all the time to decorate pots.