Some farmers will lamb indoors in December / January, but that type of lamb is usually aimed to be sold finished so isn’t likely to get sold in the store ring.
Hill farms will lamb later - not till April, many of them - and will probably mostly lamb outdoors. Many of these lambs will be sold at weaning in the store ring from July onwards, for finishing on better ground. Mostly the lambs will be on their mums for four months, maybe only three if it’s young mothers or triplets.
What you would be able to buy earlier are orphan lambs, ie ones that have been bottle reared, or which still need to be reared on the bottle. Not always the best way to start, although many people do start that way and it’s not always a rough ride! Lol.
If you ask around locally, you might find an early lambing farmer who might be glad to rehome some triplets or even take a twin off a young or struggling mum or four for you. Partly to help you get started and partly because the remaining lambs would then do better, as would the ewe. You can wean at eight weeks, so the earliest you’d get weaned lambs this way is likely to be end Feb-ish.
The other thing you might get offered is to put two or three ewes with their lambs in your field for you to keep an eye on, and to keep four of the lambs back when the ewes go home.