Given that machines that can use ADF's are all a little bit different - since the manufacturers must differentiate themselves from all the others - and that even within one company's product line the machines vary, it makes sense that the ADF's are usually integral with the machine. It's easier to design one machine with two functions than two machines that must work together. As a result, you usually have little choice but to accept the machines as they are. Nonetheless some makes have a variety of ADF's that will fit, so you can do research to see if there are benefits in alternatives. (There seem to be only a few OEM manufacturers of ADF's - that's to say, company A making ADF's that will work on company B or C's machine. There are OEM parts and to spare for all the popular ADF's; if you're sufficiently a techie you might be able to assemble one just from parts kits; if so, go to it!)
(Note that individual ADF's may have optional attachments, such as extended feeder trays or tables, or output hoppers. Some of these will be OEM products).
HP, for instance, makes the "Scanjet ADF" for its 5100, 5200 Business Series, 6200 and 6300 Professional series scanners. This is an add-on in the literal sense: to install it, one lifts off the original lid of the scanner and the ADF slips into the lid's attachment holes. There is one cable to plug in from the ADF to the scanner, and that's all there is to it. Likewise, Epson makes the B813212 ADF for its 1640XL, GT-10000 and GT-15000 lines: its installation is more of a job since it's a high-volume machine.
Much more common is an ADF being an optional extra for one particular machine. Epson makes the B12B813391 ADF for its 4490 Perfection Photo Scanner, for instance. Reviews indicate that it's a good buy for the right, low-volume environment, although it does have a tendency to run a bit too fast thus creating jams, and must be cleaned frequently to avoid image streakiness.
Kodak, Canon, Xerox, Fujitsu - all the manufacturers have something to offer. Lots of information is available on the internet, to start you off, but always verify with the manufaturer what you find out. It is best to specify the full phrase "automatic document feeder" when you do yoyr search, because there are at least 94 meanings for the acronym ADF, one of which is a popular drafting package!
A final note: while not OEM's there are many companies which will sell these devices, quite often at less than the manufacturer's retail price. DELL is one such.
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