Here's a thread I found at The Samba;
I f you going to throw away the current seats then what u do is take off all the material and strip off the springs. You'll be left with the base of the seat which fits perfectly onto the bug floor plan. then weld any seat that will fit according to width and height.
Or try http://www.cip1.com for universal seat mounts #C26-854-121 - UNIVERSAL SEAT TRACK MOUNTS - BOTH SIDES SLIDE ADJUSTABLE