Sometimes you have to call them. I have bought both of my 350's sponge strips from Needle Tek and received them very fast. They are not open on Friday or the weekends so contact them via email during those days. I just find it easier to call during the week.
I have a KX 395, which is very similar to the 350 (except that it is a 4.5mm/9.0mm gauge), and I bought my sponge strip from http://www.junebugsplace.com/index.htm. Carol is wonderfully helpful, and even taught me how to replace it.