It depends. If you are looking for paid tournaments, all rooms will have PLO. Now, if you are looking for freerolls, I think ACR offers the most. However, the GTD prize pool is small.
talking about the WPN rooms yes, the number of PLO8 tournaments has been reduced, however in recent years PLO8 cash game action from micro to high stakes, has increased.
Love plo8.
Party has a fair amount, especially at low limits.
Stars has a decent amount and decent amount of nlo8 and 5 card hi lo and courchaval. 888 and ipoker have the odd game, but not worth joining if you mainly want plo8. Bet online had a few games as well. unibet has no plo8 nether has GG.