Custom astronomical filters from Chroma and Optolong
If you want helium, neutral oxygen or lunar petrographic filters then there are two new companies that can help.
If you wanted to try out narrowoband imaging in helium or neutral oxygen bands or may be some lunar petrographic imaging then it might be easier to get appropriate narrowband filters.
Chroma as well as Optolong can offer custom filters with characteristics required for such imaging. Chroma seems to be able to offer single quantity filters, while Optolong may be more interested in high quantity (but that may not always be the case).