I used this directory from NSF to make my initial list of REUs I wanted to apply to. Keep in mind that many of these usually link to the university websites and don’t always show deadline info, etc. so you’ll have to dig a little deeper to get more details. Some programs have a unified application now, so keep an eye out for that too!
Helpful tip: if there are projects or professors who you’re interested in working with that you notice while deciding to apply, consider mentioning that (or specific aspects of the program you like) in your application! It shows that you’ve done your research and aren’t just randomly applying.
Regarding personal statements, you can find two of my past (successful) personal statements linked below.
Don’t be scared to brag or show off unique experiences/skills you have in your personal statement. This is the place to talk about what makes you stand out and what you want to gain from the program.
Helpful tip: absolutely have someone (or several people) read your personal statement! It’s helpful for catching any mistakes and making sure you’re communicating the message you want to get across clearly.
When trying to keep track of applications, I was a big fan of putting together a spreadsheet that had all the info I (and my recommenders) might need. Feel free to make a copy of the templates I created; you can find a simple version or the more complex/dynamic version I used for my grad school applications. I recommend sending your letter writers a brief spreadsheet or table of deadlines and programs as well to make their lives just a bit easier.
Finally, I recommend (with a word of caution) the Reddit, or r/REU. While there are some individuals on there who are just around to brag, I found this very helpful when I was just starting out to search for programs of interest, see other students’ personal statements, and learn more about the world of REUs. That being said, it also jumpstarted my impostor syndrome as well, especially as a first time applicant, so take care of your mental health and don’t get sucked into the negativity or competitiveness.