As a professor, I am sometimes asked to help undergrads (I'm currently at the University of Michigan) find funding for math research during the summer. In the past I have seen some get funding from the NSF REU program. The only problem with this is that it is only 8 weeks (At least at my school). Are there any other research grants that are given to undergrads that could last the entire summer?
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.
Probably you are asking about programs, rather than grants. Typically only faculty receive grants. There are a variety of search tools available for these programs. Typically the most flexible situations undergrads can get come from the undergraduate's university office of undergraduate research or from a faculty member the student knows who is willing to hire undergraduates using grant money. Most of the government research funding agencies offer a summer research program. These typically range from 8 to 12 weeks in duration and are not usually flexible, though I have heard of accommodations being made for students whose classes end late.