Try going onto IMSLP and check out vocal duets or even oboe duets.
I have done a number of things for sax and voice and they always worked out nicely.
One particular duet(s) is from Orlando di Lasso - 12 duets, really fun and they truly teach you how to be a better chamber musician, regardless of your level
Both viola and alto sax transferfrom bass clef very well, so that may be another option
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