Finding non-REU funding for my undergrad math students
I'd also ask around your department. Many universities have various summer research fellowships available. For example, one enterprising student in my lab found money through NASA that was administered through the physics department but also funded math, science, engineering, etc.

Also, most funding organizations are happy to "pad" a research grant with 1-2 undergrad research positions. I usually add these to my budget when submitting. While other areas get trimmed, the undergrad positions have never been a problem.