Hmmm, my LLS, WNNN, Puddlekins and HL have all been side snaps ... the only ones I can think of off the top of my head are the GF Utilitarian which is nippa as standard but you can add snaps... they are seriously thirsty and you can add zorb if you are getting a custom.
Otherwise perhaps Ems or Beth could do you a custom FS, I'd recommend both for heavy wetters!