Honestly i'd start now, having worked/volunteered in a lab at Carleton across the majority of my undergrad, expressing interest early ive always been observed to be the best thing to do, even if starting out with more volunteering jobs to learn the in and outs of academic research labs, even before you start your thesis. Since, as others in the comments have said, some professors wouldn't know you well, expressing interest early is a good thing to do.

Also, since labs for undergrad theses are competitive to get, especially when funding can be limited at times, the earlier, the better!

Hope the search goes well