Few garden centres stock baby leek plants, but you may be able to find them at country markets and parish sales - or perhaps you know someone who has a few to spare. Plant them out 6 or 8 inches apart by making holes 6 inches deep with a dibber (or a broom handle or suchlike) and drop a plant into each hole. Gently fill each hole with water - not soil - to help settle the roots. In time the leek will grow to fill the extra space. (Next year, if any start to flower before you have a chance to harvest them, leave them to form their big drumstick heads and watch the bees go mad for them.)
If you can't find any leeks, at least you can buy seed of a member of the same family. Yellow Japanese onions that are sown now will be mature next June or July, and White Lisbon scallions should be ready next spring.