This is a great question that highlights the differences between the halachah for a ben Noach and the halachah for Am Yisrael.
I think that it is worthwhile to ask this question l'maaseh to a posek who has some familiarity with the laws for the bnei Noach but...
While it might seem that it is safest to purchase kosher meat to avoid any issue of ever min haChai (the prohibition against eating a limb from a living animal, the "kashrus" issue for B'nei Noach), it is not clear that this is automatically the case. According to some authorities although a Jew is permitted to eat meat taken subsequent to the two simanim (signs) of shechita (slaughter) being severed, while it remains prohibited to a ben Noach as long as the animal moves (Rambam in the Mishneh Torah, Hilchos Melachim 9:12,13, the footnote in the Moznaim English translation explains that the Rashba and others rule that a non-Jew is not liable in the circumstance where a Jew isn't based on Hullin 121b).
Practically however, State and Federal laws in the United States regarding slaughter would prevent commercially produced meat from being taken while an animal is still alive. Although I'm not sure we can apply concepts such as Rov or Chazakah in the framework of the Sheva Mitzvos B'nei Noach, nevertheless it seems to me reasonable that one can rely on the government in this situation insofar as they are effectively exercising their obligation to set up "court" which prevent taking an "ever min haChai". As such it would seem permissible for a B'nei Noach to purchase pork sold commercially in the stores. Furthermore the Rambam rules (ibid 10:1) that with the exception of murder a non-Jew is not liable for an inadvertent transgression (b'shogaig).
With regard to shellfish it would not seem to be a concern because it is not clear that the prohibition applies to fish at all. The Rambam understands the prohibition as not applying to poultry/fowl (ibid 9:11). Though the Rava'ad (see Moznaim ibid) disagrees, he exempts a sheretz (creeping creature), and it is would seem seafood is just out of the equation. Additionally shellfish are often sold live, whole, or effectively whole/clearly "slaughtered". If there is reason to be concerned about ever min haChai regarding seafood it shouldn't be difficult obtaining seafood whose suitability is evident.