I'm currently using this holder. It's a little cheap but it holds the phone firmly and I only paid $1 for it. Dollar Tree sells $1 selfie sticks with that exact mount. You just unscrew it from the stick and screw it onto your tripod!

I'm using it back and forth between a small table-top tripod that I have had for years (similar to this one) and a bigger one that I borrowed from a family member.

I'm going to pick up a tripod of my own soon (this one most likely) as well as an ND filter and maybe a fisheye lens to play with.