I have not had to replace the belts on my tractor for some years and in a different part of the country. You may try these sites.

I just bought mower blades from this company and found it a a good price. (I believe they are nation wide)

http://www.agrisupply.com/garner-north-carolina/a/87/

This place is good also

http://www.tractorsupply.com/

I am not familiar with the STX46, but some belts are quite long and some are used for timing so while the ones I looked at on the above sites may be cheaper, 80 for your belt may be a deal.

Have you tried a John Deere dealer that services JD farm equipment. I have found they have prices that match the big box stores. and they know more about the equipment they are servicing.