For starters, you can always make a smaller batch, say 1-3 gallons. Pitch a single packet and ferment in your standard primary fermenter. When that is done, you have enough yeast for another 5+ gallons of lager. Or, you could make your proposed volume of wort and put 3-4 gallons aside (sanitation is key) and pitch your yeast on the remaining 1-2 gallons. Once your yeast are active, add the remaining wort and ferment to completion. I've done this small starter batch of beer for a number of years now with a fair amount of success. I used flasks with stirplates for starters before but find more value in these simpler methods.