Like calisthenics, isometrics are inherently free.
With no grasp of the concept a quick look through an old book (free) will give you plenty to test out, a solid guideline, and from there on exercise variation is only limited by your creativity with your results tied to your own personal effort.
On the creativity note the world becomes your gym.
Doorways, walls, playground equipment all become your personal free to access gym.
Everything is isometric equipment.
Besides freehand isometrics alone offer total body training. Zero equipment, right where you are, total body training. Accessable to all.
I suggest you teach yourself the usage of isometrics.
Persistence & Tenacity