It depends on your topic but what I usually do is put these words onto google

Niche "submit a guest post" Niche "guest post" Niche "guest post by" Niche "accepting guest post" Niche "write for us"

You replace Niche with your own particular topic and keep the quotation marks while looking this up so Google can give you more accurate results. You might have to do a bit of work to find some but that's just how it is. Got this trick from an old buddy of mine gotta give him credit. That's his SEO business below. gotta give credit where credit is due