At one point, l've been trying to find the same thing.
lt's really hard, the same site keeps popping up when l was searching for one before, www.pokemonworldonline.com (PWO)
l'm not sure if it's good, because they require you to take some dumb quiz to 'make sure you're human' before you register or something, augh.
Or maybe that was the site that always had maintenance in registering. Whatever, l say stick to regular Pokemon games, :)