if you are looking for an o-ring type seal, easiest way is usually to remove one and take it to the parts store and they will match it from an assortment.
Otherwise you can usually buy a small assortment of o-rings in a pack and it will usually have what you need. Here, anyway, don't know what you have there as far as parts go....
Every time I run across an o-ring in a fuel or cooling system that I don't have, I buy one of these. Has not failed me yet
http://www.oreillyauto.com/site/c/de...0243&ppt=C0142