The Society of American Florists recommends:

Look for a floral shop that offers the product mix, level of service and design style that fit your needs.

But what does this mean?

Find a florist with experience in the type of floral arrangement you need. Some florists specialize in doing large events like weddings while others focus on small Korean style floral arrangements.

Ask the florist to show you some examples of smaller arrangements or references for large events.