Money$$

I guess the same place he got the money for a $10,000 rifle...the studio who greenlighted the film. Honestly, just because you were a sniper in the military does not mean you keep all your weapons when you leave. After he left the military the guy would have very likely owned a custom Remington 700 and not some insanely expensive rifle like the one in the film. The scope you would have on a rifle like that would cost $3,500. There is a place you can buy a custom M40A3 from, but even that is $4,000 for just the rifle.

An explanation from the book not covered in the movie was that Bob won a libel lawsuit against a soldier of fortune type magazine so he had a large sum of cash in addition to his military retirement