A good place to start is the Student Conservation Association (SCA), which places over 2,000 interns a year and focuses on expense paid year round internships, many of which are outdoors. They partner with public and private organizations along with federal agencies and prescreen applicants to create a national pool of candidates for organizations to select from to bring in for interviews. Internships through SCA can offer anywhere from $75-$300 per week in living expenses, plus housing, travel and medical costs. In addition, an Americorps education award may be available to interns at the completion of their internship.
The Environmental Careers Organization (ECO) is also a well known resource for finding internships for bachelors, masters and doctoral students and recent graduates. This program partners with the U.S. Environmental Protection Agency (EPA) and through paid internships has turned out professionals in the environmental field for over 30 years. ECO selects 500 associates each year for 12-week to two-year internships. Sponsoring organizations offer internships in research and training programs in addition to office, laboratory and field work assignments. Associates earn between $400 and $800 per week and may also be compensated for relocation costs, housing, travel, and career development.
For your area of interest you can take a look in the following links:
Much success to you and good luck!