Is there any infinite source of Amity replenishment in the game that I don't have to pay real money for? A freak accident caused Roca to run away, and I don't want her to not be able to get any more Amity once I've completed all her quests, gotten and used all the Amity-raising items, and gotten her to maximum level.

There's no 'infinite' source of amity that I know of, however:
Certain events in the game (like Vacation Sweet Pack, or the Halloween event we had recently) give a small chance of getting amity+ food by using the item you randomly get from killing lvl 10+ monsters. (like a pumpkin seed that you grow into a pumpkin using turf, or the candy pack)
Another item that can give amity+ food is Riddle Paper (you get it from a quest in the fishing village of china).

Neither of these are guarantee, but if you haven't gotten the riddle paper yet, you might want to give that a shot.

Thanks. At least I know that I won't have her permanently ready to run away at the drop of a hat.

actually, you dont really need to raise Roca's amity. Because all the quests for her dont have any amity requirement, and if she does run away, you can always get her back somewhere, i.e. her house, the prison she was in, her father's grave. You will never loose her PERMANENTLY.

try going here it is cool you get gifts for leveling
http://wl.igg.com/event/gift1/

hey there is also another way for if u don't buy high price
amity jelly there are loto tickets i can buy
and go to taiwan and trade 20 tickets for 12 pet amity
on gem server the tickets run 1k-1.5k each
sorta saves money from buying and hunting down amity jelly

ya
thx sir max i will waiting next tuesday ^^

to be honest, all the amity you get from lvling is basically infinite, I mean who would really lvl up their pet to lvl 100, then reborn and lvl it up again to 199??? You will get enough amity for anything through lvling...however, this is quite time consuming, but it is worth it, because the pet will get strong!!!