Do you mean a thing you have in your hand, you want more of them?
Sometimes the thing can be 3d scanned. That process can end up being anywhere from really fast to a huge pain in the ass, depending largely on the complexity of the object and the scanning method.
As for moficiations,... that really depends. If its scanned then what you end up with is a single mesh "object" that isn't easy to modify in CAD... but depending on teh shape and such, there ARE ways.
the proper way is of course to go to teh source that made it and ask for more of them