o-rings

Where do you source your o-rings from ? That is basically my question, I guess the thickness and the quality of the rubber are important for such little things I am doing a full rebuild, originally I intended to get the head gaskets as well as front and rear main gaskets OEM and to go...

Hi everyone, Has anyone encountered any issue after using one of these gasket kits one can find on ebay or amazon ? Every time I ask sellers from where they source the content of their kit they say that they can't really tell or that many pieces comes from different brands which I understand...

I see that partzilla seems to have the O-rings and thermostat gasket-- Are there any other sources? Also, I have an Amazon credit of $50 just sitting there. Has anyone had luck ordering a kit that includes the o rings off Amazon? Thanks.