Wanted, teleconverter

Looking for a teleconverter for wildlife photography. Something can work well with DA 60-250mm (just in the case of the animals are too far). Sigma 1.4x or Pentax 1.7x in good condition at a reasonable price. I m a beginner so suggestions welcomed as well.

you might need one which works with SDM. both lenses mentioned are not auto focusing with DA 60-250mm

Woops, thought AF will work as the same, what should I go for then? BTW, will the MF still working, or they wont work with SDM lenses at all?

Tamron Pz-AF 1.4x MC4 and Kenko Pz-AF 1.5x SHQ (not DG) are the only ones that will maintain AF with SDM lenses (but make it even slower than it already is, probably not good for birds in flight in my opinion).

I have a Promaster 1.7x which has the Power Zoom contacts, the SDM lenses try to focus in AF mode with it but never settle for the correct point, going back and forth forever. Same with the different 2x converters I've tried, Tamron and Kenko, even those with PZ contacts: focus is never achieved, regardless of the SDM/HSM lens and the DSLR body.