On one of my ARs one of the gas rings is enlarged. I`m guessing from wear + tear. Now I`m not the original owner but it didn`t have this problem when I bought it. Also I have Zero reliability problems with it. Should I change them now or wait? Last where can I get them?

If you're not having any problems then there is no need to replace them. But I would guess it also depends on how out of shape the one is. Are you sure it's not just sticking up on one side and seems larger than the others. They do fit rather loosly.

One way to tell if the rings need replaced is to clean the snot out of the bolt and carrier. Then install the bolt but not the cam pin-firing pin and if the bolt will move under it's own weight when the carrier is tipped, it is time to replace the rings.

Bushmaster, ArmaLite, etc., etc. will have the rings.

ok I can check it out soon cause I always keep that area spotless unless I`m going shooting the next day or so.

From Armalite or DPMS, you can get a one piece spriral ring known as a "McFarland" ring.  This will outlast the regular three ring setup, no worry about staggering the gaps, the ring ends breaking, whatever.  Should last the life of the rifle.  

And it is only $3, which is the same as the cost of 3 separate gas rings.