You might post a free ad here on Air Forums classifieds. People are gutting vintage Airstreams now and then and you might find someone who would give it to you, but you pay the packaging and shipping. Many people just pitch their old bath parts if they are completely remodeling the interior.
You should also research and see what years had tubs that might fit your Overlander. For example, it is likely the Trade Wind, Overlander, Ambassador, and
Sovereign had the same tub and maybe for several model years. It would expand your opportunities.
David
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WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !