Try a Porcine artificial insemination site in the UK or an agricultural stock vet . I Know of one vet who obtained St Bernard semen from abroad , frozen in liquid nitrogen .
Evidently getting the bitch I .A'd was horrifically funny for she seemed more interested in eating cheese biscuits apparently than what the vet was doing .
You may find one who may have import licences as these are USA stock , couldn't find out any UK growers.