You don't even need to buy any starter, it is sufficient to buy a pack or bottle of cup of one of those 'active' fermented products - yoghurt, filmjölk, kefir, etc. Use the stuff until you have about 10% left, now add fresh (pasteurised or sterilised, up to you, no need to heat it to some special temperature as long as it is fresh enough), shake it so the two are mixed, warm the pack or bottle a bit (in hot water or in a microwave, you want it to end up somewhere around body temperature) and put it in a warm spot for a day (or longer, or less, your choice). I generally put the pack next to the water heater as that keeps it warm enough but not too hot. Once ready you use it until you have about 10% left, repeat the above cycle, use, etc.