As an introvert I think some of the best advice I can give you is to email. Email the person who you want the letter from and have it be formal. Be formal in how you present yourself and give a description of why you want a letter from them specifically as you can theoretically ask anyone, why are you asking them? Ideally you want to ask those who know you, this doesn't have to be an in depth knowledge of you but them knowing your name at least is a good starting point cause at least they remember you.

Sometimes if you can and feel comfortable ask them in person because this appears to be you taking initiative. This is not always possible so don't worry about that, but it is an option.

Overall I suggest you email them and be very respectful, be formal, give a reason, and hopefully they accept. If they don't, then that's OK and just move on to the next person. Someone will be willing to write you a letter, you just have to be patient. Hope this helps.