How can I find field work in environmental science?

Check with your professors.  They should be able to tell you about internship positions.  You may also ask them if any of them need a research assistant (since they will be doing field work as well).

At the intern level enthusiasm is just as important as how many classes you've had.

It's probably better to wait until you have a few ES courses under your belt.  That way a company will be more likely to take you on as an intern.  Then just search for environmental consulting companies in your area, contact them and explain your background (i.e. ES classes) and that you want to get some experience in field work and see if they'll let you intern.  Make sure you specify that you want to do field work, because we usually make our interns do boring stuff like filing!

I am environmental student myself and I know it is very difficult to get a job in this field due to the high job competition and lack of level entry positions (if someone told me earlier). Most of friends with same qualifications got the job through networking or by luck. I recommend training your skills where the jobs and money is in environmental industry which is mainly mining, government work (if you are lucky) and consulting. Environmental conservation is enforced by law not the good will of employers, profits come first. Choose which area to get work experience wisely.
 Good Luck

No, get field experience early and often! In addition to checking with your professors, try contacting your state's natural resource agencies (Texas Parks & Wildlife).  They may have volunteer or entry level student positions where you can gain valuable field experience.  You might also ask to shadow a field biologist for a period of time.  Also, talk with grad students and see if anyone needs field assistants for their research projects.