Are there any hechshers that certify food as kosher for B'nei Noach? Or any restaurants that offer such food? If not, how do observant B'nei Noach ensure that the meat that they eat was definitely not taken from a living animal? Presumably eating only meat that is kosher for b'nei Yisrael would be one way to do this; is such practice common among Noachides?
The Noachide community does not seem to be large enough to support "kashrus agencies" or even institutions such as restaurants. Perhaps there are some that I have not heard of (particularly the possibility of a restaurant owner who intentionally observes the relevant laws) but it doesn't seem to exist as such.
Regarding the broader issue I will recycle my answer from https://judaism.stackexchange.com/a/21964/899 which I believe has a great deal of overlap:
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 [majority] or Chazakah [presumption] 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.
There are certain foods likely to be taken from live animals and most others would not be. So for example, I'd trust that most chicken or beef available on the market is not eiver min hachai. But snow crab legs are apparently often taken from live snow crabs. So if the ben noach knows what foods are likely to be problematic, they can avoid those or devote extra caution when eating those things.
Regarding cattle and swine, the common practice for slaughtering and butchering animals involves 1. stunning the animal 2. bleeding the animal and 3. butchering (removing the limbs) of the animal. Bleeding is the process of slitting the throat and allowing as much blood as possible to drain out. So for these animals, biological death, that is death that is kosher for Noahides, has a high likelihood of having taken place before the limbs were removed. Chickens are normally bled as well before slaughter, so again, there is likely no problem. The reason for the bleeding is simple. It reduces mess/cleanup when butchering the animal. Eiver min hachai only applies to domestic and wild land mammals and birds. It does not apply to fish, reptiles, insects, crustaceans such as shrimp and lobster or even sea mammals such as whales and dolphins. So, for example, removing the limb of a lobster while alive does not fall under eiver min hachai. That being said, a Ben Noach is also prohibited from practicing tza'ar ba'ali chaiyim, that is, cruelty to animals. And this prohibition may apply in this type of case.