Hi
Firstly, thinking about SEO after you have had your website done is rather like putting the chicken before the egg. It frustrates me no end to see people pay good money for a website design and then find that they are nowhere on the search engines. Any designer/developer worth their salt will find out your exact needs for a website then either build the optimisation in during the development of the site or will recommend an SEO company who will ensure that the website (through liasing) is built in such a way as to ensure the best possible chance of good rankings.
It depends how your original designer built your site as to whether they or a.n.other can SEO it successfully. Ironically, I've seen some superb site designs that can't be seo'd effectively and yet seen some back bedroom sites that whilst looking poor from a design perspective are absolutely perfect for seo to make them a huge success for their owners.
You do not have to go to the same designers who built you the site in the first place, but it maybe that they have designed it in such a way as to make it difficult if not impossible for an SEO company to successfully achieve top rankings.
Sorry to sound full of doom and gloom but it is a bit of a hobby horse for me, as I said I find it increasingly frustrating to see people paying for websites and then as an afterthought thinking of SEO and treating it as a 'bolt on commodity', it just doesn't make economical sense.