Although the information here is written for our compact FieldPoint Modules, you can use the same products with the USB-6008.
KnowledgeBase: What Thermistor Should be Used if I Am Connecting an External Terminal Block to the c...You can really use any thermistor (even ones from Radio Shack) as long as you know how to properly scale the thermistor in software. The thermistor that you get should come with all of the appropriate coefficients.
As for thermocouples, I would check around for whoever is the cheapest thermocouple manufacturer. Definitely stick with a 'normal*' thermocouple type if you want to easily scale your date back into temperature units.
*J, K, N, R, S, T, B, or E
