Am cooking onions ring but I do not have buttermilk what else could I use

Here is an EXCELLENT onion ring recipe, and it doesn't use buttermilk:
http://allrecipes.com/recipe/old-fashioned-onion-rings/detail.aspx

It does use regular milk though, I'm assuming you have that at least?

to make buttermil, add a Tablespoon of vinegar to milk.