You can search for seo companies in google, but you should ensure that the company you are choosing uses only white hat method , else it may end up penalizing your site instead of ranking. See old work samples and results compare them, compare their response time when you contact them and ask for sample reports which they send to their client, rank sample report, SEO strategy phase wise how do they target onpage and offpage, moreover ask for their packages with detailed workscope.
This way you would select the best out of all, the company which can provide all is the one you require most likely.