If you’re looking for a thread on freewheel with 11t, you won’t find it. If you’re doing a multispeed freehub conversion and looking for an 11t sprocket (I’m assuming shimano/sram) well, you likely won’t find it. The easiest option I can think of would be to modify the standard 11t sprocket so it will slide all the way on the the freehub body and be used with a conversion kit. I wouldn’t recommend that though unless you have access to and experience with matching equipment. There’s a reason an 11t stand-alone sprocket likely doesn’t exist. Use a 12t, problem solved.