First, there's a reason why there are OIII and UHC filters. Each filter type transmits very specific wavelengths because nebulae, depending on their type, glow predominantly in very specific wavelengths. So each filter is better suited to different nebulae.
In the same thread that my first link below is a post about filters, and their use. Rather than duplicating everything there, here's a link to that very post about filters. It also details a very unique hybrid filter that is a unique combination of both OIII and UHC if watching the pennies is important, this filter is a fantastic compromise.
Nebula filters
Would even pay to read through my entire article about nebulae. You will benefit more from going through it all than just individual posts as one post leads into another and everything falls into context:
Understanding Nebulae - what it is you are looking at.
Me, my own filters are a mixed bag. I have the OIII and NPB (really just a UHC type filter) from DGM. I also have one of these OIII + Hbeta filters mentioned in the link above. All of these are 2". I use these filters when I'm using a 2" focuser no matter if the eyepiece is 2" or 1.25". I also have a GSO OIII 1.25" filter and a 1.25" Light Pollution filter that I use exclusively with scopes that have 1.25" focuser, which also tend to be smaller apertures too.
For me, if you have a 2" focuser, I would suggest sticking with 2" focusers. You will be able to wind them into the 1.25" adapter for this size eyepieces. No need to double up on both 2" and 1.25" filters.
As for Astronomik filters, they are fine. I've looked through Ivan's 18" scope with that filter. I honestly cannot see a difference between it and my own NPB filter. The Astronomik filters can be found new here in Oz. The DGM range comes from the US. Or, you can also post an ad here in the Wanted section of the IIS classifieds.
Alex.