It was previously commented by the developers but today, it was announced that you could buy the Digital Collector's Pack from the Riot Store, here at League of Legends Wiki notice that if you buy the collector's pack from the Store it is better since you get more Riot Points (unless you buy 6 five dollar packs). So if you were tempted to buy a League of Legends, your decision has been made easier. If you like to see, here is the official announcement from RiotChris:

Buying the Digital Collector's Pack just got easier.
The Digital Collector's Pack is now for sale in the League of Legends store (under the bundles tab). The Digital Collector's Pack in the League of Legends store is identical to the one sold on the website including the Goth Annie skin, 20 Champions, 4 Summoner runes and 1,380 Riot Points.
The Champions included in this pack are listed when you click on the item in the store (but we’ve also posted them here). The Collector’s Pack will cost 3,900 RP.
Q: Isn’t it odd to sell something for Riot Points that gives you Riot Points?
A: Yes it is odd, but this way you get exactly the same thing we offer on the web in the League of Legends store.
Q: If I live outside the US, can I now buy the Digital Collector's Pack?
A: Yes. You will have to buy Riot Points with a credit card or PayPal, and the prices for Riot Points are in US Dollars (but credit cards that allow US dollar purchases will apply an exchange rate so you will pay your bill in your local currency). Then you can buy any items in the League of Legends store using Riot Points, including the Digital Collector's Pack.
- RiotChris