how do i get amaster ball

You can't you only get one, so use it wisely. I would use it on one of the legendary dogs since they run whenever they get low on HP.

You can't get a secound Master Ball with out a game shark

No, unless there is a Pokemon lotory corner where you draw a lottery ticket and if it matches your ID completey you get a Master Ball but I don't think there is on Fire Red.

I think that if you breed with a ditto and have you're pokemon(not ditto) holding it it has a 50/50 chance of holding a Master Ball when it is born

Why don't you ask a nice friend to Trade you a Pokemon which is holding one?

Or, if you have two different games and a link cable, you can do it yourself...just play the game until you get the Master Ball and then give it to a Pokemon and Trade it...that way you get two master balls, and I guess you can also then boost your Pokedex by trading Pokemon you cannot catch, for example, a Bulbasaur

And with that logic, there is no reason why you cannot keep doing that...

You'll have to be patient, but if you just keep re-starting the other game, you can continue to get master balls and new Pokemon to Trade to yourself

I hope this works!

Also, if you don't want a legendary dog to run when it is low on HP, make sure you use a Pokemon which won't allow it to escape, for example, diglett or dugtrio

it is impossible. the only way to get it is to use a gameshark or if you have saffire pokemon game you can start a new file on it and get that masterball on that game. Trade it over (with a pokemon holding it as a item) to your fire red and bingo you have another Master Ball. then start your file over again and you can countinue that