What can you use to fix oil pastels?

I have heard of using spray fixers but they're not cheap. So I wanted to ask of there are alternatives? Maybe mod podge? Or varnish for oil paints? I'm new at this whole thing so I was wondering for a cheaper option. I heard hair spray messes with the colors or makes it yellow or something. And I was wondering if a liquid seal would smuge the colors. I would appreciate some help <3