2) I dont understand this question, but I think you mean is:
"Where can I buy Riot Points in a local store, not with a credit card online".
If that's what you mean.. in USA you can go to 7Eleven, and in Canada Best Buy to get Pre-Paid RP cards worth $10 and $20 worth of RP.
3) Again, I don't really understand what you're asking, but:
When you buy a champion, you own it for ever; that's all that is different. You can buy it with RP to save time, or with IP by playing games. Either way, the champion is the exact same.