What is the best and/or most economical way for a startup to setup an ESOP?

By "off-the-shelf" plan, presumably you mean a plan that your company could simply adopt by inserting its name and a few other items (e.g., share reserve, types of equity grants, etc.). There may very well be services that offer such plans. But it's probably not an optimal practice. The laws that apply to stock-option plans vary from state to state, and an OTS product may not address this nuance. Further, while their terms are fairly standard, stock-options plans can be complicated to operate.

Typically law firms and perhaps compensation-consulting firms offer model plans that can be easily tailored to a company's needs. Plus, those types of providers would usually be in a position to properly advise on the plan's operation.