It depends. If your purchase is to be from GSA, a commercial vendor, under the Simplified Acquisition Threshold, and/or with the Government Purchase Card, then FAR parts 12 & 13 might help clarify your options (FAR part 12.212). If however the purchase is to be part of a larger acquisition and might entail the need for rights in the data, then assistance can be obtained in FAR part 27 (FAR part 27.405-3). Additionally, you will need to be cognizant of the requirements of your agency relative to method of funding used to make the purchase to ensure you don't violate the Anti-Deficiency Act. Additional assistance may be gained from your contracting officer and program legal representative.