An internet search will turn up some information.
Gun shows, firearm forums, on line firearm auction sites such as gunbroker, auctionarms, or gunsamerica.
Provided you are not a convicted felon, you can buy long guns at 18 and hand guns at 21.
I don't think you have to have a license to work on guns, but I'm not sure about that. To manufacture guns, you have to have a manufacturers license. Check with the BATF for more information.
What kind of information?
One would be able to find out more information about m4 airsoft guns online on Amazon. Two other sites that have some information about m4 airsoft guns are wikiHow and HobbyTron.
You can find information on when these guns were manufactured on OldGuns.net.
Look on Nerf.ea.com to find out more information
Information on 45 caliber guns is available on websites that sell guns, for example Cabelas', and also websites of arms organizations such as the National Rifle Association. Information is also available at the website The Gun Zone.
Branquart guns are high-end Belgian 'best-quality' guns and need to be individually appraised.
Trade name used on guns sold by Farwell, Ozmun & Kirk of St Paul. Manufacturered by W. H. Davenport, Crescent, and probably others.
Information about impact guns can be obtained by visiting any local or retail fire arms dealer. The sales representatives are knowledgeable in the different types of these items and can answer any questions about them.
Crosman is the current Owner of Benjamin. You can find out some information on www.Crosman.com under customer service. However I suggest that you get the book "Bluebook of Air Guns 7th edition" and in the Crosman section & Benjamin section you will find your answer.
More information about DeVilbiss spray guns can be obtained from the official DeVilbiss spray guns website, There is a resource section located on their website which offers a wealth of information about DeVilbiss spray guns.