Few years ago when I worked in a physics lab we used some very narrow (sub nm) filters for our experiments. They were not astronomical filters and the transmission was usually about 50% but the price was much cheaper than the comparable astronomical filters.
For example, 1nm FWHM 50mm diameter Ha filter is US$424
here.
And if that is not narrow enough,
here is 0.15nm FWHM 50mm diameter Ha filter for US$2078.
The tradeoff is that you may need to double your integration time.