You have three options here:
Use the_post_thumbnail()
You say you’ve already done this with one theme. This is really the best solution because it keeps everything using core code (which is frequently updated) rather than third-party libraries.
Upgrade TimThumb
Since the “most recent” issues, they’ve released a new version of TimThumb that corrects the security vulnerabilities. Merely replacing the old system with the updated, secure one should be the quickest path to resolution.
Email the developer
If you’re using freely-available themes, you’re out of luck and might be stuck fixing things yourself. If you actually purchased the themes from someone else, please email them and ask that they release an update.
It’s not really their responsibility to provide a security update, but most quality developers will jump at the chance to update their code to prevent huge security holes. Besides, it never hurts to ask.