If you can't find rings that are made for Tikka, try CZ they will fit if you leave the locking tab off the back one.
Or you could do as I did on mine and used Weaver two piece bases and rings of the proper hight. The reciever is already drilled for the Weaver bases.
I would rather have the Tikka rings, I feel they look more sanitary, but like you say, they can be hard to find.