The Parker and Cross are fairly standard, easy to find at Staples or Office Max type places, sometimes even Walmart. Honestly though, the best refills I have found are the gel refills (Parkers) that CSUSA sell, and they are cheaper than "true" parkers by a longshot.
Rollerballs are a different story. There are as many rollerball styles out there as there are religions. The only rollerball refill I can find at local office stores that fit our rollerballs is the Pilot G2, and I only use the "fine" point, b/c the "extra fine" is just too skimpy and writes terribly. I cannot find a "med" tip G2. Most people will know the name "Pilot G2" and recognize it as a good writing refill.