Sanur is where most Ops launch from to dive Nusa Penida. There may be a few from further north near Padang Bai but time/distance to Nusa Penida would be increased a bit. Penida is where you'll find Manta Point (for mantas obviously) as well as Crystal Bay (known for Molas) and a load of fun drift dives. Sanur has a good selection of beachfront hotels to choose from at a variety of prices, including a Hyatt if you're looking for more western style accomodation. Blue Season Bali, Crystal Divers and Bali Scuba are the 3 big Ops in town but there are probably 10-15 other small name ops as well. Without knowing your exact preferences, those details should get you some fairly detailed results with a quick google or tripadvisor search.