You can always make a filter out of Baader solar filter material. You can order it and they will send you directions on how to make the cell to fit your telescope...it is very easy to make. You can order it from many places...here is one.
Baader solar film order
An astronomy club I once belonged to found a place that sold 5inch square sheets for a lot less. A 5 inch sheet would be adequate for a DS90 filter, but I can't remember who we ordered from.
Rob