There is one UK zapper manufacturer with a range of zappers -Commercial Science. They make standard and higher Voltage Clark Zappers for more penetration and also the Becks zapper.
I have the latter which works ok. I have nothing to do with these people and am just answering your question. They are not cheap and we are talking UK prices here, nothing cheap. You will find many good zappers on sale from outside the UK but then you may pay customs duty and it almost evens out. Your choice.
I tried zapperplans 'zappers'. They are no more than a cable with 2 crocodile clips on one end, a battery Holder on the other for the 9 volt battery. In a clever rubber sleeve they house the components to reverse the polarity and create the =positive offset effect so it works as a zapper. Your obviously going to use up a number of 9 volt batteries in this without the basic Clarks circuitry in a box like the ones above and all others on the market. The orgone zappers seem to be pretty good, and the Terminator 2 is good Worldwithoutparasites.com and batteries last for yonks.
If your just experimenting then get a Zapperplan or the other one mentioned. If you have a chronic health problem I would invest in a proper zapper whoever you get it from.
Hope this helps.