There are several places you can buy an army bomb. I'll be showing some examples.
Our shop - ARMY may have questions about us selling legit army bombs, to answer your questions, yes we do. We are the distributor, getting official BTS & bt21 merch from the manufacturer.
Weverse shop - The most reliable way to buy an army bomb, which is run by bighit. The tricky thing you have to download the app and the price may be expensive.
Concert site - Whenever BTS has a concert they have merchandise booths and you can usually buy an army bomb (and batteries) there as well. The price is also expensive than online merch and AMRYs have to wait hours to get products.
Amazon - You must have seen an army bomb sold on Amazon before you were here. The price on Amazon is the cheapest among the three places that I have listed above. Take army bomb ver.4 for example, the price is $44.99, I am not sure where did they get the product that can be sold at such a cheap price. AMRYs can read reviews before taking the next step.
To be honest, ARMY bombs are not cheap! Most of the time they (army bomb ver.1-2) cost $30 to almost $60. So if any seller is selling them from $10 to $25 dollars, there's a good chance that it's a replica, or this person is being very generous, lol, I talked too much.