Not sure about buying them for credits, but I know you can use 2nd Order Exploration badges to buy them at Earth Spacedock (2nd Floor).. I've been farming Hromi exploration missions to outfit my science vessel with new weapons.
Exploration points are easily worth it. You get nice exp also and its quite fast. IM harvesting MArk IV gear myself at the moment. You need to be lvl 12 though to get exploration missions for mark IV stuff.
Keep in mind some of the higher weapons have rank limits on them -- I learned this the hard way. When you hover over a MkIV weapon it should say, in green, on the right hand side, the rank required to use that weapon. So don't bother spendign the energy credits (or anything else) until you are, or are close to, that rank.