I am not quite sure what you are asking. Do you mean where do you purchase one or what do you look for.
If you mean purchase one, then I suggest several of the airgun web sites. Competition biathlon rifles can be very expensive. An example is the Feinwerkbau model P75 Biathlon that cost around $2,500. Today most, not all, of the biathlon rifles use a PCP system (PreCharged Pneumatic) With the required support system this will add to the cost See the links below and when they open look for biathlon models made by Feinwerkbau, Air arms or Airforce. There are also others that you may want to examine.
You shoot a target with a rifle
The equipment you need includes skis, poles, a biathlon rifle, standard velocity ammo, a biathlon sling, and a harness.
.22 long rifle.
X-C skis, .22cal rifle
rifle basix makes the trigger of your dreams..
One of the best is a Feinwerkbau Westinger & Altenburger. Biathlon Air Rifle Model P700 Aluminum. Average cost $2,480. It's an Olympic Match Rifle. The best air rifles for target shooting are Olympic Biathlon match rifles.
Cross country skiing and rifle shooting.