Lynne, you can make the ham without the saltpetre, only it won't be a pleasing pink colour - it'll be greyer.
http://www.naturalcasingco.co.uk/ and http://www.sausagemaking.org/ are where virtually every home curer in the UK gets their saltpetre from, however. It copsts around four quid for half a pound, which is plenty. Get some suplphur from the garden centre and grind up some barbecue charcoal, and make some proper crackers with what's left over!
Be careful if you're tempted to buy readymade curing mixes from the sausage man - some of them are based on potassion nitrite rather than nitrate and thus have a shorter shelf-life and (because the nitrite is toxic) you have to be more careful about quantities.
Even the nitrate is bad for you in large quantities - the FDA in the US outlawed its used in bacon in 1989.
That said, nearly all bacon in the UK is made with it and saltpetre, as E252, is approved for organic bacon.