For a global Find and Replace, if you export the SRT, you can then open it in and use any text editor's Find and Replace function.... then resave (with the SRT extension) and bring it back into Resolve.
Appreciate that's a bit of a rubbish solution, but so far BMD haven't prioritised this sort of thing with their dev team.
If you need a quick 'built-in' find function for subs, I wrote a helper script which you can get from the thread here:
Creating Scripts for DaVinci Resolve