I have to find a supplier yet as well.
For pig semen, I only know of Deerpark.
http://www.deerpark-pigs.com/They don't list all the breeds that they have on their website - give them a ring to find out what they can offer you.
They are very friendly and helpful and their service is excellent. Just be aware that although the semen will keep for up to a week, they don't tell you not to put the box in the fridge if you are storing it for a few days before using - it needs to be stored in a cool but not cold place. (I learned about this the hard way...
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I don't know the Yorkshire breed - do you mean Large White? If you are wanting to AI, Deerpark have Saddleback, which is a good bacon pig so wouldn't be too lean, and I think also Welsh, which is quite lean for a traditional breed I think. (But any of them seem to be able to put on fat if fed plenty!
) They don't yet have an OSB boar up and running but are hoping to get one working soon. According to their website they also have Hampshire (robert will tell you how this would be as a cross), Duroc, Pietrain, Landrace and Large White.