In Pokémon XD, Gligar, Vibrava, Skiploom, and Quagsire can be obtained via Poké Spot, either by direct catch (in Gligar's case) or by evolving a Pokémon caught there (in the other three cases), being Shadow Pokémon in Colosseum.
Additionally, Meditite, Tyranitar, and Shuckle can be obtained in both games, being Shadow Pokémon in Colosseum and being able to be traded from Duking in XD (and Larvitar being leveled up and evolved).
In the official Nintendo Power Players' guide for Pokemon XD: Gale of Darkness, there is a screenshot that displays Shadow Ralts at level 22+ rather than 20+. Another screenshot in the same guide depicts the Shadow Nosepass at level 40+ rather than level 28+. Either these screenshots are mockups or some pre-release versions of the game had certain shadow pokemon at higher levels than the final version.