I don't know about best sources, but it's pretty easy to find a list of symptoms and an online quiz (Google "testing dyslexia" or something similar), which would give you some sense of whether to pursue it further.

Are you already accepted to a program, or still planning applications? If it's the former, you can probably contact the university about what their requirements/recommendations are for their Office of Disability Services. They may even be able to offer referrals (local to that area, most likely, but it's a place to start).

If you're still planning applications, then I'm not really sure where you would start - a pdoc would be one option, but there are probably others. You might be able to make a call to the pdoc's office just with the question about getting tested for dyslexia and get a referral without anything complicated, but it obviously will depend on your pdoc. Since dyslexia is a learning disability, it seems like that's usually handled through various complicated educational system stuff, rather than various complicated mental health system stuff, but I might be completely wrong on that!

I didn't find any good quality resources on a link between dyslexia and bipolar, so it looks like either it's not been investigated much or maybe there's not much of anything there. It could just be lousy luck to have the two conditions!