We believe the part you are looking for is a stem raiser. Take a look at this one, the Delta Cycle Alloy Stem Raiser. You'll want to take into consideration the length of your cables and housing, as sometimes raising the stem will require you lengthen them. Although it is about a kid's bike, this thread, Riser for kids handlebars, has some more good information regarding this item as well.
Hope this helps!
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