Filters and galaxies
Nebula filters work by cutting out the majority of the optical spectrum, but transmitting those wavelengths at which nebulae glow.
Thing is with galaxies, they glow across the entire spectrum, and do not have the same spectral bias as nebulae do. As a result, nebula filters will not help with making galaxies easier to see.
If you have a scope 16" and larger, there is one thing that a nebula filter can help with in galaxies. There are a small handful of galaxies that have massive glowing hydrogen regions. These immense nebula regions are also massive stellar formation areas. A UHC type filter can help reveal these regions in these galaxies. To have a chance to spot these very difficult features, a dark sky is not only required, but also very good transparency. Unless transparency is as good as possible, your chances to spot these very faint features is next to none.
"Galaxy filters"
There are some filters that are marketed as being "galaxy filters". Some are produced by very reputable manufacturers such as
Omega Optical (link). I have no experience with these, so I cannot comment on them. Maybe someone else here has, and can offer some comments on these filters.
Alex.