If you have an upgraded membership (Pro Event or Pro VIP), you have the ability to export/download your donation details and donor company contact information for approved donations. Simply go to your "My Events" tab and click on the appropriate "Export" button for the event.
If you have a free membership, you may use the contact information they chose to provide when approving a donation or send a direct message through the donation record to ask for contact information or send a thank you note.
The majority of companies we work with are simply happy to support organizations and prefer that organizations not spend the time or money on thank you letters if not required to do so per tax law in your state or country. A publicly visible social media shout-out/thank you is appreciated more (Facebook, Twitter, Instagram), and we forward you a cheat sheet for making it easy to do so prior to your event for this exact reason. Most companies also appreciates recognition by digital means such as mentions in email newsletters, websites, social media, and any collateral for the event.
As a bonus, thanking donors publicly is a very important step of being a good partner and may even help you secure more items in the future when potential donors see that you actively promote generous companies.