http://www.oselectronics.com/ose_p119.htm#Speakers
I bought a few speakers here. Very good quality and cheap price.
Oh, okay. Just measure from one side of the frame to the other side. The ones listed are that way. I restore the old Hallicrafters radios, and the S-38's use a 5 inch speaker. Drops right in, and can use the original nuts.
measure diagonal your square mount frame. That is how the square ones are shown. If it is 3 1/2 they don't have that one. If that's the case I can check with the old radio shop here for what you need.
Another http://www.wjoe.com/stand.htm#speaker