Google shows me that there are at least 7 garden centres round Leicester, plus an indoor market, an outdoor market and a farmers market. If they're plantable now I'm sure one of those will have some for you.

But I think you have missed the boat for this year. You needed to plant out the small plants in May or June.

Agree with Beattie it is too late to be planting out Brussels sprout plants now. Welcome to GoY Gingernutty.

Ah, you've now joined GoY - welcome Gingernutty! I assumed you meant Leicester in England. Perhaps you're in Leicester, Vermont? Probably even less likely to get sprouts there now.