The grips that fit the RTI, are the same as the AR15 powder fire gun.
Don't get the ones with the "tail" on them. You'll just have to cut it off. Also, on some brands, you may have to do a little "fitting".
Many go to a lesser angle grip, all the way to 0 degree. There are a lot to choose from that are about 15 degree.
I've got three at 0 degree and two (Daystate uses the AK47 style) at 15 degree. Makes a big difference in your wrist angle.
Optics Planet and Midway are good places to see what's out there. Most anyplace that sells the powder fire guns will have grips. Be sure to look for the angle or the term, "anatomical".
Mike
P.s. - You really...don't NEED...to do anything, but fill with air, a sighting system and pellets. Suppressors, grips, various sighting systems, extra feet, changing colors, etc., are all extras that aren't needed, but many like to add to their guns.