There are a few ways to go about it. MyCokeRewards.com allows you to redeem coke points for RP cards. I believe for 300 points you get a $5 RP card. Another option that I use is Bing Rewards. You can use Bing Rewards to earn points and redeem for gift cards. There are no League gift cards on there, but you can redeem certain other gift cards for RP through the Client (such as Burger King, Sports Authority, Subway, etc). I use Bing Rewards and get Burger King gift cards to redeem for RP. The other simple one out there is Swagbucks. It's similar to Bing, but different as well. You can redeem swagbucks for Paypal gift cards (that you can use to purchase RP through the client). Also, Shopkick is an app that allows you to get points for walking into certain stores and scanning items with your phone in the stores. You can earn gift cards by doing that (Sports Authority is the one that you can redeem for RP through the client).
Those are the only legit ones I've discovered (and I've used all four). In my opinion, if you drink Coke products, Coke Points is the easiest, then Bing Rewards, then Shopkick, and then Swagbucks.
Also, if you are not aware, you can get Tristana (and her Riot Girl skin) by liking League of Legends on Facebook, Garon (and his Dreadknight Skin) by following on Twitter, and Alistar (and his Unchained skin) by following on Youtube (I might have mixed those last two up), but you can get those three champs and skins for free by doing those things.
Hope that helps!