Originally posted by Pioneer I use the Pentax 1.7x and it seems to work fine, but that one is not available new.
It is if you order it through an Amazon trans-shipper in Japan (Google them - some have websites in English). And it's an excellent TC that will add semi-AF capability to MF lenses too.
In addition to the above AFA x1.7 I also have the Tamron x1.4 Pz-AF MC4 which is also excellent and fully AF compatible. You may find though that for 90% of your work (if for birding or wildlife) the DA*300 is so sharp that just cropping is almost as good as a TC (certainly a x1.4 or x1.5, the AFA x1.7 certainly edges cropping at 510mm though).