Where on the tax form would I record that expense?

I would think supplies or you can enter it in one of the blank sections that you can name.  Maybe call it samples?

In TurboTax, simply search for "Schedule C" and then click the "Jump to" link in the search results.

Then (assuming your business has already been set up in TurboTax), you'll want to click Edit next to your business and then scroll down to the Business Expenses topic on the Your Business summary screen.

Otherwise, if you haven't set up your business yet, follow the onscreen instructions and eventually you'll come to the part where we ask about your business-related expenses.