Window locking pins can be almost anything. I drilled a small through the window frame, main window casing and fixed window casing. Then I just stuck a nail through it. It prevents the window from moving at all. I preferred my method with nails because it is cheap but very effective. You cannot lift the window out of its track while open.

You can buy ones with a sliding latch that screw into place on the window sill. If the latch is a bolt that slides into a hole you drill into the window casing, you could have several positions (closed & 2-3" open) it could lock into place. Some also have keyed locking pins.

Keep the placement of the locking pin at the back of the window opening. And don't allow the window to open too much. Just because you may not be able to reach though the opening or have long enough arms to reach the pin from the outside, it doesn't mean someone else won't. There are some long skinny armed folks out there. Anyway, be conservative on how open you place your open pinned position.