Your "Looking for a power sprayer" has been closed so I will answer
here. "O" rings are VERY standard. Search your Yellow Pages for "O"
rings, Seals, or "Parker" O rings, or Grainger.com or GOOGLE
McMaster-Carr or go to mcmaster.com and you can find tons of the
things. Measure, with a ruler that has sixteenths markings, the inside
and outside of the cavity your ring fell out of and buy that ring. It
will be 1/8" diameter or 1/16", buy one of each. Or take your sprayer
to Grainger or some similiar commercial parts supplier in your town and
they will fit it for you.
It will cost less than one dollar.
If you need further help leave another reply here.
Joe Till
Houston