Oops!
I really must learn to read the headings as well as the questions ;-)
It's about 30 years since the last cameras using an M42 screw fit lens were manufactured. (The secondhand Praktica camera I bought around 1970 used the M42 mount but when I bought a new Praktica camera a few years later, they'd changed to a bayonet fit). So it's extremely unlikely that you'll find anyone still manufacturing them.
An additional problems is that, at the time M42 lenses were manufactured, zoom lenses were very expensive (and consequently rare). Most amateur photographers would use a standard lens with a tele-convertor (and possible a wide-convertor).
As Paul suggests, eBay might be the best place to look but if you're lucky enough to have an independent photographic dealer near to you, you might get a secondhand lens on the high street. (You might find that your only option is to buy a virtually worthless camera, such as a Praktica Nova 1, to which someone has attached a short zoom).
The only suitable lens that I've found on eBay is this one:
http://cgi.ebay.co.uk/28-70mm-300mm-T2-M42-scr ew-lenses-for-35mm-Camera_W0QQitemZ14016817187 4QQihZ004QQcategoryZ4688QQssPageNameZWDVWQQrdZ 1QQcmdZViewItem
Chris