Sorry, no direct link as this is, afaik, not in production any more. Keep an eye an Ebay and also KEH.COM has sometimes this teleconverter. Price is between 100 and 150 euro.
Please pay attention: there are several kinds of KENKO TC's and only one, marked with powerzoom, is the one which supports SDM lenses.
I know several people using this in combination with a 300mm zoom. Personally I prefer always a lens without TC to one with TC. I my view a TC takes half a stop light, has difficulty focussing in low light and mostly is less sharp than a zoom. I use the TC for "emergencies" or in combination with a prime.
In this topic
Het Nederlandse PENTAX forum - Kenko 1.5 TC op DA*300 a forum friend has posted some photos of the KENKO in combination with the PENTAX DA* 300mm prime.