Yes there is, its called a product key. You can buy them online or in a store on general release. You can also get keys next month if you have a Technet sub. If you install it now, you could rearm it until you do have a product key.
Any other ways are not for discussion in this forum.
Because as i read i can't upgrade from RTM to final, so if i install RTM i won't be able to upgrade to the one that i buy. I know that i can activate the final one with the serial I buy, but.. can i activate the RTM with that serial?