One can find numerous online agencies that offer English to Spanish translation services. They constitute a mixture of free and paid services. Of course if you are looking for professional quality translation then you should go for a reputed paid service, because free services often produce incoherent translated documents. 

What you should check out before hiring the services of a particular agency is the quality of writers it has on its rolls. A high standard agency would have a mix of academically qualified and/or sufficiently experienced translators. They would also have provisions for proofreading, this means that the company puts in the best efforts to ensure that the translated document is error free and of a high quality.

However every company might not offer proofreading services, moreover those who do offer such services, would likely charge an extra amount. So it’s up to you to decide whether your budget allows for the extra charges of proofreading services.

A benefit which is mostly offered by Spanish translation companies operating online is the availability of native Spanish translators. The advantage of having native translators work on your English to Spanish translation is that they would be well-acquainted with the nuances of the Spanish language and would be able to tailor the translation for maximum authenticity. However, don’t go by this criteria alone as its always possible to get excellent Spanish translators who are not natives but are capable of producing high quality English to Spanish translations.