The easiest way to find puzzles where counting the number of triangles is the goal is to search online for them. Because math, children's books and plain triangles come up when searching either counting or triangles alone, it is important to search for both words.
When searching for triangle puzzles, adding the keyword puzzles to the search will produce more creative responses than the typical triangle within a triangle response. Because it is a visual puzzle rather than a word puzzle, an image search will also provide a larger number of puzzles. Some of the simpler puzzles will come with answer keys.