The Tamron TC is actually very low in price compared to the Pentax, being the reason for the feeding frenzy.
Pentax came out with one a long time ago and you can rarely see these ones for sale.
The Pentax ones can make a MF lens AF, the reason also for fetching a ridiculously high price.
There are other alternatives apart from the OEM Pentax (which is no longer in production) and the Tamron (which I think is also no longer in production, being also the reason for the high price).
You can get the Kenko 1.5X
There is also the Promaster 1.7X
Members here avoid the 2X since it greatly degrades the IQ and speed of the lens.
Pentax:
Pentax-F 1.7x AF Adapter Teleconverter SMC MINTY 1.7 x | eBay
Kenko:
Kenko 1.4x Teleplus DG AF conversion lens for PENTAX | eBay