Hi David,
I would recommend the DGM Optics NPB filter. This is also sold under the brand name of Omega Optical. It is classified as a Narrowband (UHC) filter but it has a very tight band pass and is almost a hybrid UHC/OIII filter so you get the best of both worlds. In addition to this filter I have the Astronomiks UHC and OIII filters and the Lumicon UHC and OIII filters. I have also used all of the OIII and UHC filters from Orion, Celestron, Meade, Thousand Oaks, Televue and Baader. The DGM Optics NPB gets the most use of all my filters. It offers a noticeable gain on a larger number and wider range of targets than any other filter I have ever used, probably because its almost a hybrid.
http://stores.ebay.com/OmegaFiltersE...&submit=Search
You can buy them at a good price on EBay but do not think for one minute they are a cheap inferior product because you are buying from Ebay. There are several other vendors who also sell them.
Cheers,
John B