You need one 25x stack of each green, blue, and yellow salvage kits + 3 mystic forge stones to create a 250 use Yellow Quality Salvage kit in the mystic forge.
To get the salvage kits for Karma go to a Weaponsmith Trainer (Master Weaponsmith) which I have highlighted in the picture below. You can find them on his 2nd bag
Then You will need 450gems to buy 10 mystic forge stones from the in game shop. As this is the best return on investment.
400+gems for 1g is a great conversion rate.
MAP OF LION'S ARCH