This is probably the easiest thing for anyone to donate to the fleet provided that you are a level 10 or higher character. I say this is easiest mostly because you can do it with absolutely no real world or in game cost. It is also is rather profitable with 300 fleet credits for every white(common) DOFF donated. Please however remember that any DOFFs that are donated to the fleet will be PERMANENTLY removed from your roster so don't donate them unless you can afford to loose them. DOFFs are available for free from the 3 cultural exchange missions (Vulcan, Tellarite, and Andorian) at Starfleet Academy as well as 4 missions from the Personnel officer (note she has five missions but General Recruitment costs 1000 Dil). Romulans can run these missions at the faction of their alliance as well as at the command center on New Romulus for an extra helping of 4 Doffs from a Personnel Terminal. Of course like anything DOFFs can be bought & sold on the exchange for anywhere from 12-40k E.C. for white(common) (apparently in the future slavery exists… just kidding). Another way of easily obtaining DOFFs is from your fleet. At the starbase you can purchase DOFF packs with 500 fleet credits for a pack that gives you 1 random common duty officer or 750 fleet credits to get one common duty officer of guaranteed specialty (science, engineering, or tactical). Fleet projects normally only need white(common) quality DOffs and if you have extra of higher quality DOFFs that you don't need there are two ways to get rid of them. The first being the exchange if the DOFFs are not bound. The other way to get rid of high level DOFFs is to turn them into low level DOFFs. You can do this simply by talking to the Personnel Officer at the Academy and selecting "Exchange Officers" for trading in extra green(uncommon) officers you will get 3 white(common) officers. For trading in a Blue(rare) Officer you will get 3 green(uncommon) officers. And for trading in a purple(very rare) officer you will get 3 blue(rare) officers. Officers received in trade this way are random and not guaranteed to be of any specific type, but trading in officers in this manner also rewards an amount of Recruitment XP depending on what quality officer you trade and is often worth it to get rid of extra DOFFs.
Side Note DOFFing is an often overlooked aspect to the game for new players but your DOFFs and the missions that they complete can be one of your best friends in STO. Make sure (if you want to be powerful in game anyway) that you have your active space and active ground slots for your DOFFs filled with DOFFs that provide you with some helpful powers. This is one of the secrets to some of the best builds in the game.