There are many internship opportunities available to someone with such a broad scope of interests and pathways as you have shown interest in.
Broadly, large law firms and Big 4 accounting such as Russell McVeagh and PwC do tend offer some limited opportunities for final year high school students, often accompanied by helpful follow-on university scholarships. The standard for these types of scholarships is certainly high with a rigorous application and interview process, but they are still much more accessible than equivalent opportunities overseas. Look at the PwC “School Scholarship” and “Accelerate Opportunity”, and also “School Leavers’ Scholarship” at Russell McVeagh.
If you want to differentiate yourself, you could seek out overseas programmes such as those offered through organisations like Sage Corps, where you can complete internships at start-ups and gain a greater insight into the workings of a smaller business. Alternatively, in many areas of law, it is common to practice independently. Taking the initiative to reach out to someone you admire in this field could create a uniquely insightful experience through which to gain some much more hands-on employment over summer.
If you are involved with any school branches of organisations like Amnesty or MUN, your school contacts may also know of both international and local opportunities that are available to high school students that year also.
Best of luck!