So i need 22 RP for a champion and i dont want to spend any more money just for 22 rp is there any way I can get 22 rp?

You don't need RP for champions... I highly recommend using your IP for that, and saving RP for skins.

I have heard that Riot will occasionally grant you the difference for a purchase if that difference is small, so you could try sending a support ticket explaining the situation, but don't count on it.

Ultimately, the best solution is to purchase more RP, since presumably you'll want to make other purchases after this one.

how do you get a support ticket?

In the upper right of your screen, next to the logout button, you'll see a SUPPORT button. Click it. I believe the ticket is in the upper left of that screen.