I buy a sachet of culture from winequip, about $40, but I do some red wine as well so it's worth it for me. If anyone knows of a cheaper source it would be good to know. You only need a tiny bit of culture for cider, the recommended pitching rates are for wine but cider has more malic acid so it is easier to do in cider. If you have contacts at a winery they might give you some, you only need a pinch rehydrated in water at 20C.
With the right temperature and low so2 a mlf will happen naturally in cider, I pitch the culture to get it over early so I can bottle. I don't know if it is worth bothering for supermarket juice. The MLF takes about 2 weeks, then another 2 weeks for the flavour to settle down, initially it will taste very flat after MLF but soon improves.