Do you want a M5 (non-Free Floating) RAS? If so, these should be available most anywhere. If you are looking for an SR-25 FF RAS tube... well that is a whole other ball of wax! I have been watching to find one for years and I have yet to get my rather gready little hands on one... I know that a few were sold over the past couple years but I hear it is easier to find a hen with teeth then to get your hands on an SR-25 FF RAS still!
Now, if you have not seen this review...
http://quarterbore.com/forums/viewtopic.php?t=86... check it out now! Art (AKA SniperX) has an exceptional tube in this setup and frankly I like it better then my FF RAS tubes (My AR-15 FF RAS tubes that is). The ONLY disadvantage is that the specs are not true Mil-Std-1913 so some accessories will not mount but I have been using my AR-10 Carbine with carious ARMS style mounted equipment and I really love it!
Also, you will note I moved your post to the AR-10 forum... no offence just trying to keep everything where people can find it...
If you manage to find a source for an SR-25 FF RAS please let me know as I would love one for my AR-10T...
See That Gun here if I got the chance not that I don't like either of the tubes that are on my AR-10s...
EDIT... I see you said SR-25K... that will be even harder to find... you might try contacting KAC and see if they will do a factory upgrade as a carbine length SR-25 FF RAS is something I have never seen sold by itself... Art's system should work for you however as your SR-25 would look like my CAR-10 project at the first link!