There are several ways to go about doing this. In any case you should definitely purchase the font before using it in a final product. Creating quality fonts can take a lot of time.
The first way is to find the font online for sale and use the foundry's font previewer to test out the words you want to use and see how they will look.
The second way is to use the foundry's website to create a preview for the font and then download the preview as an image from your browser to use as a comp in your design. (You can also vectorize the downloaded image using Illustrator or Vector Magic)
The third way, which is questionable from a license standpoint, is to find the commercial font online for free download. Once you have tried the font, make sure to purchase it legally before using in any final product.