You could make your own spacer using PVC Pipe. I have made some for Handlebar and Steerer Stem. Cut the length of PVC you need and then cut it in half lengthwise. I had to put it on a grinding wheel to get a perfect fit. Make sure to wear a dust mask when grinding.
Sometimes you can get headsets with a lower stack...it won't be much.
You can get riser bars and twist them a little forward.
Thanks gents. I found my stem ...Sunline V1 XC Stem with a 33mm stack height and 90mm long.
AS, just a suggestion. The bmx star nutz tend to be twice as strong as the standard ones. This may be something you want to consider.