Citizen Bike (https://www.citizenbike.com/catalog.asp?product_category_id=1 https://www.citizenbike.com/catalog.asp?product_category_id=...) has 20 inch wheels.

Citizen Bike (https://www.citizenbike.com/catalog.asp?product_category_id=1 https://www.citizenbike.com/catalog.asp?product_category_id=...) has 20 inch wheels. Not ultra-compact, but the one I have is pretty good.