The TTL-232R-5V will work with the Atmegatron.
***Please note, I now recommend buying the cable from the Soulsby Synths store: http://soulsbysynths.bigcartel.com/product/programmer-cable (http://soulsbysynths.bigcartel.com/product/programmer-cable)***
Here's some links to where you can buy them:
Farnell (http://uk.farnell.com/ftdi/ttl-232r-5v/usb-serial-cable-vdrive2-vmusic2/dp/2419945)
Mouser (http://uk.mouser.com/ProductDetail/FTDI/TTL-232R-5V/?qs=sGAEpiMZZMtf8aSAtgR7DpPMdam%2fwe00)
Digi-key (http://www.digikey.co.uk/product-detail/en/TTL-232R-5V/768-1028-ND/2003493)
Adafruit (http://www.adafruit.com/blog/2010/04/13/usb-ftdi-ttl-232-cable-5v-ttl-232r-5v-in-stock-and-shipping-immediately/)
Sparkfun (USA) (https://www.sparkfun.com/products/9718)
Cool Components (UK) (https://www.coolcomponents.co.uk/usb-to-serial-ttl-cable-oem-ftdi-cable.html)
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