I have been doing this for years. I began by buying long hair wigs, not ponytails. Several years ago, the long hair wig market disappeared, so I began buying ponytails.

At first, I bought shorter ones, 18-25 cm, but as I gained knowledge, I focused on longer and longer tails.

Now, my minimum length purchase is about 130 cm. Weight is also important, as well as texture. Be prepared to spend a lot. A 140 cm, 500 gm tail will easily cost in the neighborhood of $1000.

Prices on eBay vary widely. A short tail may set you back $50. I sold a 46 cm tail for $149. There are two long, heavy tails currently listed, each for $2700. Way over priced, and they have been there a long time. They probably will not sell.

Shop carefully, do your research, and be patient. You will eventually find what you want.