As the others say store lambs are usually had late autumn time, those that have not made a good slaughter weight. If you specifically want sheep to graze your grass now then it would most likely be ewes with lambs, although these wouldn't be too cheap. If you could stand bottle feeding you could get some cades, although even these tend not to be given away free any more! A good source of sheep for sale is on Preloved, you'd get an idea of what's available and the cost. Commercial cross sheep are generally cheaper than the purebreds, but a good bet if you want to breed your own for market later.