Players. You can return the vial to magic shops for 5 gp.
Looks very similar to Wine. Before the 8.1 Update, you could buy manafluids in magic shops for 55 gp each. This is no longer possible, since they are replaced with potions. The only way to get manafluids is from Daily Respawns or from players that still have some bps of them.
See the comparison table mp/gp for more information.