Any good after market antennas for this radio?
Yes but by no means are they unique to this radio. The Diamond RHF40 or RH77CA comes to mind...
I also heard you can put an amp between the base of the radio and the antenna. Is this true? Also is their a chance of frying the radio?
You can put an amp on any transmitter. I wouldn't keep the same antenna (the rubber duck) though, obviously.
You can always fry a radio. Depends what you do to it. I wouldn't stick a paper clip across the antenna terminal and transmit for an hour. You could put it in a pan and put it on the stove, too.