You only ever receive one in every game. But I would suggest Dusk Balls if you're trying to catch the rest of the legendaries. I still haven't used my Master Ball but the largest number of Dusk Balls I've even had to use on one Pokemon was Zygarde, which took me only 3 attempts. Dusk Balls are $1,000 each.
In BW/B2W2 there was a second way (forgot how, but i do know I have two unused master balls for each, in case I ever found a shiny). One I believe was given by Professor Juniper and the other from Colress. Now, as for X/Y, I'm not sure if you can get another one, I haven't found a second yet :).
i need another master ball like really bad... well i need it for my brother. he's been trying to catch mewtwo but ran out of money and pokeballs. the only games of pokemon we have are x and y so i need to know if i can get another. we both used ours on our ledgendaries.... now i wish i hadn't but i still caught mewtwo AND yveltal, my two favorite ledgendaries. yay!
The last post in this thread was way back in October, which was 5 months ago. Please do not revive topics over a month old, otherwise it is considered topic revival and is against the rules. Also, things like what you're asking for should be asked in the Giveaways and Services subforum. Thank you!