The pig and its all kinds of products are haram in Islam thus must be avoided. It is not permissible to use any piece of pig when producing something. However, we live in a global world which is very difficult to avoid doubtful things. After chemical process and mixing in other elements the pig products become doubtful, the believers should keep them selves away from doubtful area so they can protect their religion and religious life. As for your question, if you know that shoes has pig product in it then you should avoid it, but if you did not know until you already bought and started to use it than it is ok to use. Because, the essential theme in the prohibition of pig is its meat and its consumption by human beings. However, the scholars deem the pig najis (filth) as a whole and prohibit it as a whole without classifying according to its usage or consumption. The problem here is what happens after chemical process and mixing with other elements. Additionally, using a shoes which contain a bit pig product becomes doubtful but not haram.