Sigma offers 8 thrombin substrate products:

http://www.sigmaaldrich.com/life-science/biochemicals/biochemical-products.html?TablePage=15845126

You can choose based on what type of assay you want to do, e.g. absorbance or fluorescence. Minimize the cost by minimizing the assay volume, using low-volume microplates. Fluorescence-based assays can use much less substrate (e.g. 1 µM) because the detection of fluorescence is much more sensitive than absorbance. But also check the literature to find the Km for each substrate. If the substrate has a very high Km, you will have to use either a high concentration of the substrate or a high concentration of the enzyme, resulting in higher cost.

Misha

http://www.sigmaaldrich.com/life-science/biochemicals/biochemical-products.html?TablePage=15845126.

Thank you, Saranya,

the article is very helpful