You are best to get three sheep for a small group - they are flock animals so if one dies, you still have two, but if you only have two and one dies then it's a mad panic to get a suitable extra sheep so the one left is not alone.
Cade lambs may often have health problems with not having been brought up by a ewe. You are better to buy your sheep as weaned, direct from a sale such as the rare breeds sales which are held between August and November each year - you can get pedigree rare breeds there and unregistered animals too.
More likely you will be looking for wethers if they are just to keep your grass down rather than breeding from - first identify a person who keeps the breed of sheep you want, (doing the rounds of the agricultural shows is one way, though not in 2020), and has a good reputation for stock quality, then approach them for available stock to buy. Wethers will be lambs destined for meat, except in a few rare cases, so you need to buy them as lambs before they reach the age for slaughter (which varies with breed) I wouldn't just buy unknown animals from an unknown source just because they are available when you want them - plan to buy healthy animals which you have examined in person. Equally, don't be a time waster - if you have chosen your breeder, then be prepared to buy when you visit. This visit allows you to make up your own mind about their animals and husbandry - if you are unimpressed then say so early and leave.
Above all, don't just rush into buying animals - take your time to find the right breed for you, the right breeder and the right animals.