Through trial and error I have found the best place to purchase these pens is www.jetpens.com They have the best prices, amazingly quick service, and free shipping with a $25.00 order. Be careful though, I have found that it is often cheaper to purchase the pens individually rather than in a set, prices are slightly different. Maybe they will change that in the future. I first found out about Jet Pens through Joanne Sharpe and she was so right. Give them a shot and see if you aren't equally happy.
Curious about your use of white opaque pens?