You can get two more from Mephistopheles if you do her side-quest in order to get the Foe's ring, unless you let Biorr die in 1-4 in order to get his armour, which in that case, you'll only get one Colourless Demon's Souls
Here's the Side-Quest and what you'll have to do in order to get the rewards: http://demonssouls.wikidot.com/npc-mephistopheles
I'm not a fan of using Heavy Arrows, I tend to use normal arrows, since they are very cheap, thus I didn't have to spend much souls on them. Plus my Lava Bow took care of all the rest xD
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