It's tough, since one isn't made specifically for this size BU any longer, though antique chipboard cases frequently come up on eBay for ukes with a 7" pot. Do what Mike and Kimmy suggest; your best bet is to look at new hardshell cases that fit sopranos and just try for the fit. My Slingerland 24 fit into a Levy case for concert wooden uke just perfectly, though I never tried them for sopranos.
Lucky you - I LOVE model 18s.