Recommendations for 1.4x teleconverter

I want to get a teleconverter to go with my soon to arrive 70-300 Tamron. The Sony ones at $500 are out of my price range. Can anyone recommend a good reasonably priced one under $300 USD. Tamron unfortunately doesn't make one for the Sony mount.

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Tom

Look at the picture, not the pixels

http://www.flickr.com/photos/63683676@N07/
http://www.flickr.com/photos/25301400@N00/