I have a DIY CA reactor and can't really comment on commercial ones but as to your question about refilling CO2 tanks. I have mine filled at a commercial gas place that services food companies. You can also check Beer and Soda distributors, they usually will exchange empty tanks for full for a fee. Also, some Beer making supply stores will swap tanks as well.
One think that you'll problably hear a lot about is people getting them filled at paintball places. I've heard that some of these places add an oil to the CO2. I can't confirm it one way or another but I'm not taking any chances.
I got out my yellow pages and called some beverage and beer making stores. They referred me to the place I go now. I asked the guy there to confirm that it's food grade CO2 that he uses. The bottom line is Quality. A little dust, oil or dirt in the gas might not hurt the typical application but I don't want to risk my investment.
As for cost, refilling a five pound tank around here is about $11-12. A ten pound tank is about a buck more. I bought a 5 pound tank off of ebay for about $50. If I had to do it again, I'd buy a 10 pound so it lasts longer.
