Cartridges for CO2 Inflators

I’ve got an Innovations CO2 inflator. The directions say to always use Innovations cartridges but one of the guys at the LBS suggested I could get cartridges cheaper at Wal-Mart. After I’d used one or two, the inflator stopped working. Anybody had any experience/luck with off-brand cartridges?

No problems with the Walmart variety on my Innovations inflator. I’ve used only a couple times at most though.