Government Lab Internships and Jobs
Pathways to Science is the most general search engine. It may overlap many of the other links below, but it can return a lot of programs that may not be of interest to you. One nice feature is that it allows you to segment the search by education level.
USA Jobs is general search engine for federal government jobs. Many of the government labs and agencies handle their applications through this website.
Here are some individual federal laboratory sites. Some may route you back to USA Jobs, but looking through the actual sites will give you a better idea of what they actually do.
The US Department of Energy's (DOE) 17 national laboratories are excellent places for summer internships. I would encourage you first to apply for their Science Undergraduate Laboratory Internships (SULI). Applications need to be submitted by January 9, 2020. The applications take some time to assemble, but, more important, you should do some exploration of the research areas at your target laboratories. If possible, find an alum who works at that lab to advise. There may be some opportunities available at individual labs after that date, but the SULI internships are most attractive to the researchers within the lab.