I think it's already been mentioned that you have to be prepared for not all the lambs being ready for the off before the abattoirs and butchers close to home use intake - which is end of November around here and in Cumbria.
But you can always buy in hay if you've had to keep a pair or three back for a bit longer
. Presumeably you're buying hay for the miniature horses anyway.
As to buying lambs in spring, if you want to buy them in spring, as I was trying to explain, you will need to find someone who is lambing before Christmas and is willing to sell some lambs as stores. Depending on location, lambing in winter usually means the farmer is aiming to fatten all his or her lambs themselves and sell them all as fat / finished in summer, so then they may not want to sell them as stores.
The other way to get lambs in spring is to be prepared to take orphaned lambs and bottle-feed them yourself. If you're livestock-savvy, that might work for you. Expect some losses, even if you can find a farmer to reliably only sell you the very best, definitely had a good bellyful of colostrum in the first 6 hours of life, lambs. And of course, you do get very attached to them when you bottle-feed. (Even wizened old hill farmers do.
)