There are instructions on Dr Clark's site to build your own Zapper which should be very reasonable. I don't personally know anyone who has built one but it may be worh a look. I have also heard of someone finding one on ebay.
Also on Dr Clark's site is a formula for parasite cleanses but there are a number of companies making a Wormwood and Black-Walnut hull product with or without cloves. My personal opinion is that the Wormwood and Black-Walnut hull is best taken as a tincture and the cloves freshly ground and encapslataed at home. Doses will depend on the strenght of the tincture.
For any other Brits out there www. herbshandshealing.co.uk do a good parasite cleanse formula (and also have a lot of useful info on the site) and Zappers can be purchased from www.traditionalhealth.co.uk.
Perhaps those in the US and elsewhere can help with where to purchase elsewhere?