I found a bunch at Rock Auto - check Trico in the part catalog search section. There is a method to their part numbers - numbers ending in 150 is 15", 160 is 16" and so on. I think the last sets I bought began with 43, like 43-150, 43-160, etc.

TRICO Part # 43-150 Long Frame Metal Refill; 15" (380mm) For Trico and Anco Long Frame Metal Blades
Category: 15" Blade Refill