Answer:
There are several places to get one. Depending on if you want a production rifle or a custom rifle that will change where you purchase it from. If the gun is production it will be from Korea (That's a very good thing) and usually less powerful than a custom made gun.
If you live in America you can visit http://www.pyramydair.com/ to buy one. They carry one and its called the shinsung career dragonslayer .50. They also sell other big bores which may suit your needs.
If you live in Canada you can buy it from Air Gun Source (see related link for website)
These guns are pcps. Meaning you have to fill them up with high pressure air (3000 psi)
Which requires a special handpump or scuba tank.
Custom guns can be purchased from a few custom manufactures.
Denis quackenbush
Gary barnes
Big bore bob
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The above answer pertains only to air rifles, which the question is not limited to.
.50 caliber rifles (gun powder) are commercially available firearms and are purchased just like any rifle such as a .30-06 hunting rifle. Best place to go is a gun store.