Might go to Lippert website, they have a lot of diagrams with parts numbers available, or call them with your pump number...
I am dealing with the same problem. They not only did not have a replacement o-ring available, they could not tell me where to get one or exactely what size it is. Lippert's response was to soak and clean the existing o-ring and use Permatex red or orange to seal. I did that and it did not work. I found a fatter o-ring at Napa and installed it yesterday, it still leaked. I'm now looking for an even fatter/deeper o-ring to try. I may resort to making a gasket myself as I'm tired of taking it apart, dealing with the ATF cleanup.
Seamus
Normally, they handle customer requests better then this. Have you considered taking the o-ring to a hydraulic shop? It has to be oil-proof to last, so regular rubber o-ring won't work for long even if it does stop the leak.
Or, call RollingRetreats in Elk City, OK. Slade or Jason may be able to help. Pump model # will also help.
My new pump is installed and there are no leaks. I now have a spare motor that will disappear into my shed never to be needed again. The financial hit is upsetting but I'm more peaved that Lippert could not, or would not be more helpful. I did not have a faulty pump, I had a deformed reservoir/collar. They would sell a complete unit pump/reservoir, or a reservoir with no collar and noSo far this summer we have had 34 days over 110 degrees and I've run out of patience working on this in the sun. We have reservations later this month for the Grand Canyon which will be in the low 80's and we need to cool off. Lippert wanted $1125 for a complete new unit, I found it on Amazon for $900 and bit the bullet, it will be here tomorrow,