So essentially, what you would want is an Arduino pro micro with a usb adapter. Seytonic on YouTube did a whole series about a DIY rubber ducky using an Arduino. you can find his first video on the subject here but once you have seen said video I recommend watching the updated video on the subject which you can find here. He also goes deeper by adding on different functions to the Arduino like SD card storage or switches to change between scripts.
While this is one of the cheaper solutions it also means that you have to do more work to get it up and running. If you are not versed in the arts of Arduino programming, or do not feel like putting in much effort I recommend you go check out Maltronics and go to the MalDuino section. The site is hosted by Seytonic himself and all of the products are hand soldered by him. I believe this is the best option for your needs considering that you do not have an extremely low budget. The cheapest option the MalDuino lite costs about 16 USD, and to convert between ducky script and Arduino you can check out Seytonics DuckyScriptConverter.
One other thing you can do, although I advocate against it, is to go out and buy a bunch of DigiSparks for one dollar each. This is probably a bad idea unless you have hours to spend fiddling with them like me, also most of the ones that you will get will come in broken which is why I recommend you buy a considerably large amount. But like I said, unless you have time to spare, I suggest you stay away from this option.