Cool, a fellow Britt. Welcome to the board!
There are many ex railroad, mining, and farming communities near Bozeman, some of which are no longer displayed on highway maps which you can purchase in stoors in the UK or the US. Perhaps compare modern maps of the Bozeman area with the 1895 US Atlas of Montana (accessable using a quick google search for '1895 US Atlas') to get an idea of which towns exist, and which don't (in the sense that they are no longer inhabited, but there may still be remains to this day).
If you have the option of travel, you may wish to consider exploring the neighbouring counties which border Gallatin County. If you have this oporunity, keep in mind that travelling around Montana's roads (when venturing away from Bozeman, Helena etc.) is far easier when in comparison to British roads - there will be no traffic jams, and barely any traffic for that matter!
Another option is to purchase a good book. Such items will be easy to locate at Amazon or even eBay. 'Lonely Planet' guides are pretty cool and make great travelling companions for tourists from overseas (me, for example!), but they don't delve particularly deep into the Ghost Town side of Montana.
Check my Montana pages for the counties which surround/near to Gallatin County. Just a note, my MT pages aren't quite complete!