Instead, ask everyone at all levels. Some of the best ideas will come from your lower and mid-level employees who are interacting with your vendors, customers and co-workers every day in the very areas that, if improved, could make the most dramatic impact.
Be sure to acknowledge receipt of each idea. Keep everyone informed of the types of ideas you've received. Perhaps update them on a monthly basis. When you implement a suggestion, recognize and reward the submitter, including possibly financially.
Feel free to accept ideas anonymously. But, if employees know you are sincere about wanting their input, and witness you acting upon suggestions, most of your team members will be proud to tie their names to their ideas.
Finally, if there are some of the same suggestions that are offered up by multiple people, odds are those are the ones that need your immediate attention.