For the moment RV Surplus
has what appears to be the right sender for those tanks, their SKU g12845, for $24 plus shipping (and these guys are
a little high on their shipping charges). The snap-on gauge they carry was the right one for my recent-model Manchester tank.
My 2-cents' worth: if you think you'll add a monitor later, go ahead and get the tank with sender installed. I looked around a lot before finding mine at this site. Manchester Tank told me to go to my local propane suppliers, and the locals basically told me to go fish! (Note that the remote sender has a gauge face with pointer that you can read at the tank.)