Hey how do you get a febass. P.S is there another masterball..

By fishing at route 119. Do every tile 2 times with the old rod. You will eventually catch one. It may take you 10 seconds to 1 hour to finally encounter one. Hope you have enough time on your hands.

Feebas.
Yes, Feebas is available only on route 119 by fishing with ANY rod at least 5 times on every square (tile) of water. I say 5 times because you CAN catch Magikarp, Tentacool and Carvanha on the same tile. It's a HUGE hassle.
BUT, here's the twist: Feebas only appears on 6 (yes, SIX) tiles of water in route 119. That's a pathetically small number of tiles out of the hundreds of tiles in Route 119. AND (if that wasn't irritating enough) these six tiles are changed everytime you cange the 'Trendy Phrase' in Dewford Town. So good luck....

Alternatively, if you don't want too much trouble, trade one from another game and breed it to get tons of feebas.
As a last resort, the AR/GS/CB is always there...


Now for your second question:
Master ball.
The only existing Masterball is in the Aqua Hideout, in the room with 4 pokeballs in it. Two are Electrodes, one is a Nugget and the last, the Master Ball.

o o <-- Electrode
o o <-- Electrode
^
Master Ball, Nugget

So if you mis it, too bad. HOWEVER, don't lose hope. You can still win a Master Ball. Everyday in the Lilycove Department Store, the lady on the right behind the first floor counter lets you draw a Loto Ticket. If the number on it matches one of your Pokemon's ID (in party or PC), you win a Master Ball. Seriously.

The last Master Ball I know of is on Mirage Island. If you're lucky enough to get there, go to the plot with the Liechi Berry in it. Use the Itemfinder and.... Ding! It detects an item! Search the spaces around the berry plot by pressing A repeatedly and a Master Ball will be yours.

Hope it helps!