To employ a professional Spanish translator can be be a pricey business, especially if you have a lot of material to translate.
On the internet, there are software translation tools you can use. Some of these are for purcahse and download, while others will do the job (for short passages) on the fly. The trouble is some of these tools tend to do a word for word Spanish translation, which can at best sound stilted, and at worst can produce complete gobledegook!
The better tools will naturally have a better stab at Spanish translation, but they can never replace the authentic touch of a fluent Spanish speaker, who understands the complexities of the language, and who is at home with its colloquial expressions.
So if budget is an issue, you need to decide how great is your need for accuracy. If your project could survive with less than perfect Spanish translation, then perhaps machine translation is the way to go, but if you wish to put out a prestigious publication then it has to be human Spanish translation every time.
Of course, if you have some facility with the Spanish language yourself, then you could, if you have sufficient time on your hands, undertake your Spanish translation yourself.
In most communities, you will nearly always find that there is either a native Spanish speaker, or someone who is fluent in Spanish. Ask around! Your friends, neighbours, and colleagues are bound to know of someone who can help you.
If you go down this route, prepare a rough draft of your material, and then work through it with your Spanish speaker, paying special attention to passages you are unsure about, or unhappy with.
You can also find a lot of help on the internet. A search on Google will bring up an absolute plethora of sites where you can do some research to iron out the more difficult passages in your text.
Also keep an eye out for forums when doing your research. In most forums relating to any topic, there are always plenty of friendly, helpful people willing to share their knowledge and expertise.
Good luck with your Spanish translation!