I'm writing a solar system simulation that uses n-body physics. To plug in the celestial bodies in our solar system, I'm copying orbital values from wikipedia. For one example, the wikipedia page for Europa lists the periapsis distance, the apoapsis distance, the mean orbit radius and the average orbital speed. I'm currently plugging in the average orbital speed and mean orbit radius, as using either the periapsis or apoapsis in combination with the average orbital speed would result in inaccuracy for bodies that have eccentric orbits. Unfortunately this solution does not also seem to be especially correct, as the average orbital speed is not always the speed that a body travels at the semi-major axis.
I would rather starts the simulation with each body at it's periapsis or apoapsis and having the corresponding velocity. Is there an online service where I can fetch these orbital speeds, or is there a way to calculate them?