Some acoustic focused amps would have a separate xlr input for a mic.

If you don't have one of those then you're pretty much out of luck. Mic and instrument level are different things, so your amp won't be set up for the mic signal level.

I'd go for a cheapo separate setup, either a dedicated mini PA or run the mic into a mixer and out through some speakers. Also be aware that some microphones require a powered connection to operate, although most all mixers will have at least one channel that can do this.

IMO the best bang for your buck mic is a Shure SM57. You can use it for everything from vocals to an amp to a snare drum.