This, IIRC, requires an older version of 1Password. Version 4, from the "Who moved my cheese" discussion of 1Password's most recent subscription changes.
I can confirm this option still exists in 1Password 6 (I'm using the iCloud sync but I see the option for dropbox as well). I'm not sure how to get the license though. I only see the upgrade path that I took - not the outright purchase.
You can still purchase a Mac license there. I don't see a Windows option, but I'm viewing this from a Mac, so not sure if they're just hiding it. (Some people in this thread have said that there's no longer a non-subscription version for Windows.)
FWIW You can do this with 1Password. Preferences > Sync > Sync with Dropbox