does anyone know where I can get a 爆撃機 ジャケット like Americas?????

Hmm lets see... Expensive way:Ebay または something like that. Hard way: Attempt to make it または find someone to make it for you.. または the Easy way! lol Find a cheap jacked that looks close to it and duct tape the 50 to the back of it! I was close to doing that myself XD Its cool that あなた have the rest of the outfit :P (not the best answer but its some choices)

If your actually buying the cosplay outfit itself, try ebay, または アマゾン (I haven't check amazon, but i'm obsessed with amazon. it has almost everything...)
But あなた may find this shocking, but あなた can actually find a bombers ジャケット like Alfred's at the mall. try checking the stores. But really, my friend dressed up as Alfred with searching handpicked items, she got a bomberjacket at the mall, went to a fabric store, got the number 50, got a star, and a airplane, amde out of patches, put them on. And it came below 25 dollars.

I saw some on Ebay.

cosplays aren't always that comfortable though online...I'd say it's better to find a 爆撃機 ジャケット style ジャケット at clothes stores and add the 50 on the back, the 星, つ星 on the left front side and the airplane on the left shoulder/sleeve... I helped my friend with her America ジャケット (her sister helped as well). it looks quite amazing!