Voltage divider but don't know current requirement, so how to pick the ratio
For an RS232 level converter the current drawn will depend largely on what it's attached to and the line state at the time. If you use low value resistors you'll end up generating a lot of heat / waste power and higher values won't have good regulation as the current changes.

A linear regulator such as the L78L33ACZ is in a hobbyist friendly TO-92 package and is only 40 cents in one-off quantities from Digikey. The drop-out voltage of 1.7V would put it just within spec. I'd suggest buying some of those or something similar from a local supplier and purchase a few extras for your parts drawer for future projects.