In the UK most factory tours seem to involve either a) people related in some way by business (clients here for a meeting, why not take them round the factory), b) student groups, c) family/friends or d) the local members group for professional institutions (eg in the UK the institute of mechanical engineers). I'd imagine your best bet is to see what the latter groups are doing (although whether or not you can attend without having the right degree is probably fairly dependent on local policies and how many other people want to attend).