How about putting post cards in corner shop windows in the areas you're looking at? A lot of people have small "lock ups" or garages or workshops or outbuildings, but they're often pretty worthless and would only rent for £50 per week or so, so the estate agents just aren't interested as there's not enough profit for them, hence they are just left to be full of junk or ignored.
I "found" my own office for my business by word of mouth, after asking around friends and family. It's a basement of someone's house, but on a slope, so it has it's own doors and windows around the back which is ground level back there. Previously used by a builder as his yard/storeroom. Owned by an old couple who lived in the house above, and been left empty since the builder moved out a few years earlier - they just couldn't be bothered instructing an agent and dealing with the insurance/lease etc so for them it was easier just forgetting about it. Once I showed interest and promised to help them sort out the formalities, they were happy to let me have it, and then a few years later I bought it from them!
Just by looking around, noticing empty premises and doing a little digging and asking around, I found a small shop that had been empty for a couple of decades and also a small joiners' workshop that had been left to go derelict - that was a shame as it had loads of stuff in it, old tools, old signs, etc. that would have been fascinating to look through. Neither were advertised anywhere, and owners of both were open to discussion, but both were a little to derelict for my needs.