If you're asking about Victorian Britain I looked into it and gave up, that's why I decided to go for 20mm. That way I could tap into the world of HO/OO modelling, there's more Victorian Britain architecture in there than you can shake a stick at. I got my 20mm Prussains from B&B Miniatures and will be getting my 20mm Brits from Newline (wee bit of conversion needed) and hey presto I've got England invaded with all the terrain I need. Alternatively if the invasion is the other way around there's plenty of Continental train modelling manufacturers who deliver the goods.
Then again depending on the size of your 15mm figs they may fit in with HO/OO scale buildings anyway. I've got some 15mm Lancashire Games figs and they are huge, pretty much 20mm, even bigger when you base them!
There is always the alternative of scratchbuilding.