Since I started this wesite AudioMoth has expanded there are now four variants, AudioMoth, HydroMoth, DevMoth and μMoth. I am only covering the AudioMoth.
AudioMoth is a low cost wide spectrum acoustic recorder, it is a research tool and as such is not available through a shop. If you want one read the manual and if you still want one go to GroupGets or the Open Acoustic Devices forums website and see if a buying round is open. It does not come with a warranty or a case. Because it has a wide acoustic spectrum it is used by groups and individuals to record a variety of sound including gunshots, cicadas, birds, tiger roars and bats.
As of November 2018, on a GroupGets buying round they are $49.99 plus shipping which is a tenth of the cheapest recorder I can find. There is now an option to buy an AudioMoth at $69.99 plus shipping.
This site is focused on bats and I'm assuming that you are already into bats and bat calls, so there is a LOT of information I'm missing out.
If you have found this site and are baffled by it - I suggest you head over the the Bat Conservation Trust where there is a lot of information about UK bats.