To begin with I'd say is that if one must take Miracle-Mineral-Supplement , then I'd highly recommend buying the components to make it yourself. This way you don't have to deal with older solutions or mixtures, and have a better idea of what you're getting. However... if for some reason that's not an option, then I'd recommend buying some of the pre-mixed solutions from a site such as eBay(ie. Sodium Chlorite or water purification kits). At which point, you should do your best to find a seller with good feedback.
Secondly, if you're taking MMS, I'd also recommend following the baking soda recipe used to adjust pH and correct some of the effects such as; nausea and other potential stomach problems as shown here: http://www.voiceofmms.com/fe/21286-mms-basic
And finally, I'd add that MMS has recently been preceded by a new product based on the same chemistry(Chlorine Dioxide) called CDS - Chlorine Dioxide Solution, which is(in my own experience) much improved over the original product called MMS. That is to say that it has nearly no taste and side effects(tiredness and nausea) and more importantly, allows for larger initial doses without any of the side effects associated with MMS, which in-turn allows for a far more rapid response and deeper penetration.
PS. I haven't read or heard any conclusive evidence that MMS/CDS can help eliminate Candidas, though I have read that it can help with the effects(or toxins) created by Candidas in the bloodstream.