There is a lot of difference between Beer Stone and a Calcium (usually Carbonate) buildup. You really need to identify which problem you have.
A Carbonate buildup can be removed with most any acid. Just make sure it isnt strong enough to harm you, your brewing equipment...
Beer Stone is quite a lot more complex, you really are best advised to use a made for the job chemical from someone who really knows how to manage the problem.
Beer stone is a blend of both Calcium Oxalate (mostly from malt) and Proteins (mostly), the Proteine protects the Oxalate from some acids, the Oxalate protects the proteine from Alkaline (NaOH) cleaners.
Bit of an Advertorial, but this from
Morebeer isnt too bad a start, covers how to and a bit of the chemistry behind how it works.
Mark
Someone like Kegland should be able to order 25L of a dedicated Beerstone remover and repack it. There are some that might even not need to be shipped as DG, worth asking around some of the better chemical manufacturers, preferably one who specialises in brewing solutions.
Mark