Your chapter will get support from speakers, sponsors, etc. Do not hesitate to keep a more or less informal list of people to thank. E.g. R-Ladies Philadelphia keeps “a Google spreadsheet listing name, support type, email address, postal address, who’s responsible for sending it, and status of the thank you card (sent/not sent)”. Of course such data must comply with personal data protection rules.
Thank you cards could be sent once a year like R-Ladies Philadelphia does (“we try not to splurge, but try to add a sticker or other small token of appreciation as well when possible”), by snail mail, or more regularly by email. Remind recipients on why you are thanking them, especially if it has been a while since they helped or collaborated with you.