Where, or from which monsters, do unique/rare plans for rare/legendary/set items and gem designs drop in Diablo 3?
Everywhere. From every monster of an appropriate level for the pattern in question.
Aside from the components for the Staff of Herding, the Infernal Machine plan and keys and a handful of named Magic Items which are essentially easter eggs, there are no specific drop tables tied to any specific mob or location in Diablo 3. Everything is completely random (modified by Magic Find/Nephalem Valor as appropriate).
After reading through the D3 forums it seems that they are just as rare as legendary items, some people claim to have gotten them in the later acts of hell, most say they've found them in inferno, one person claimed to have gotten one for a level 21 bow in Act IV normal. So my best guess would be the later acts might have better drop rates, also MF supposedly helps in finding them as well.
If you make inferno butcher runs, you will start piling up quite a few plans. I have found at least two of each of the following exalted plans: sovereign mail, sovereign helm, pallium and dread shield. Each of these are mediocre quality rare plans which sell for about 5k each on the auction house, so it's not like you get all happy about finding them (except when I found the first one and thought it was awesome). If you want to find any of the good plans I'd bet you have to search in inferno act 3/4. If you just want easy plans, grinding butcher runs for some hours is probably the way to go.