Google "livestock market" and a location
Just did "livestock market northants" and got
http://www.laa.co.uk/market_details.php?RID=210who appear to do pigs on saturday.
I would repeat my caveat in the other thread - you can get a bargain or a problem, and if you have other pigs, isolate any you buy in for a few weeks to reduce the risk that you don't introduce something.
Yes you can in many cases you can salvage a poor specimen, but having had a bad start it can take a long time and lots of food to get it to slaughter condition. In some cases if its initial growth was stunted or it had a disease that checked it, it will never make it fully up.