Look in the yellow pages. Ask to see quality-control manuals. Describe your problems or research needs, and ask the research companies to submit proposals along with costs. Typically, proposals are free. Look for quality of thinking.
Chemistry is important.
Honesty and integrity are supremely critical. Based upon the visit, the chemistry, and the proposal, choose a research company and give them a small project first. If they do a good job, then trust them with more important jobs. Never, never choose a research company just because its prices are the lowest. Let them experiment on your competitors.