I want to also thank Michael for the information about mixing potassium oxalate, and for the link to the Chemistry Store. Sounds like a good resource.
A couple more notes about sodium citrate and potassium oxalate. With kallitype there is virtually no difference between the two developers so far as I have observed. When the image is toned with gold, palladium or platinum both developers give the same tone and image color regardless of developer temperature.
With palladium there is some difference in results because if you use potassium oxalate at a very warm temperature the color of the print becomes much warmer. With sodium citrate the look is almost identical regardless of whether you use the developer warm or at room temperature.
BTW, it is also possible to save money by mixing your own sodium citrate, using sodium carbonate and citric acid. However, since sodium citrate is already fairly inexpensive I have never bothered to make the mix.