do you mean a forwarder or a channel with the emulator inbedded in it? for the first i think most forwarders already work even with the updated emu. for the second i think you would have better luck making a channel yourself then finding one for download

 

there isn't an official channel installer for genplusgx or else it would be here

http://code.google.com/p/genplus-gx/downloads/list

 

Those aren't full channels.
They are called forwarder channels.

There are loads for Gens gx around the net.
This is a nice one: http://dl.dropbox.com/u/13773490/Megadrive%20forwarder.wad

Make sure the .dol you are using is'nt compressed.
( don't download via Homebrew browser )
Get the official version from genplus gx download
http://genplus-gx.googlecode.com/files/genplus-gx-1.5.0.zip

Rename the included Wii.dol to boot.dol and replace it in your apps>genplusgx folder on you sd card.
A forwarder channel is better becouse you can update it more easily. ( just replace the boot.dol in folder.)
genplus does'nt have an auto update feature ( yet ). So it's the best way to go if you ever want to update.