We bought this high quality (heavy duty) swing seat, and made an A-frame since we didn't have a tree to hang it from. Kids have outgrown, so now for sale. Remains in excellent condition as the seat is weather proof and made of high quality material.
70$ for swing seat plus A-frame and hardware, or just take the seat if you want to hang it from a tree.
for details on the seat itself, you can see it here:
https://www.amazon.ca/dp/B005KUKJOW/ref=cm_sw_em_r_mt_dp_U_elJhDb8GANP30