One of the local irrigation dealers carries what they call  'irrigation batteries'  intended for power units.  No warranty.

  They appear to be  gp 31's   with studded terminals.   These are   'remanned'  (whatever that means)   and are sporting  brand new cases.  

These were 50 bucks  last month,  so I'm sure I'll  see an increase soon.
  and  like you,  if  I get 2 yrs out of them,   (which I normally do)   I'm ahead.