measure .. there are Tube type stem raisers in .833" and 7/8" AKA_22.2mm.
you pull the stem , put the tube riser in, tighten the quill in it,
then put your original stem in the top of the tube riser , then tighten it's quill bolt.
then you replace all the cables and housing that will likely be too short, then.
If there are spacers above the stem, you just take the stem off (remove the top-cap, loosen the bolts on the back of the stem, take it off, and move the spacers that were above the stem down below the stem, and the reassemble everything).
If there are no spacers on the steer-tube above the stem, it's probably as high as it can go. The only option I can think of would be to get a new stem with a larger angle (or, possibly, you can flip your stem, depending on what angle it's at now -- here's where a picture could help).