Probably your closest LDS is going to be Groundhog Divers out of Port Elgin (I think they are just off of Hwy #21 and 2 weeks ago when I went through on my way to Tobey they had a big sign pointing the way to the store).
Not sure what the diving is like right around there but if you want something idfferent then Tobey the best wreck that I know of (the lake bed in that area is basically sand/mud/silt) is alomst directly out of Grand Bend (bit of a drive) and it is called the "Wexford" - amazing wreck since it is in great shape although I would suggest to be at least AOW before doing this wreck - in about 60-80' with at least 1 and sometimes 2 thermoclines. If you want more info on this wreck check out
www.shipwreckwexford.ca for the history of it. I have dove it 4 times this year and this coming weekend we are headed back out to it. There is also supposed to be some diving sites around Godrich 0 I think I have heard of the area being referred to as the boneyard sue to some really broken up wrecks but have never dove it. There was also some talk about a ship being sunk on purpose around Godrich but apparently that has been put on hold. If you are AOW the PM since there might still be spots available on the charter this weekend.