What can you use instead of Cointreau?

There is no difference between Cointreau and Triple Sec. Cointreau is simply a brand of Triple Sec. Triple Sec, therefore, is the generic term (like refrigerator) for liqueurs such as Cointreau (which would be equivalent to a fridge). Even though Combier claims to be the inventor of Triple Sec, the origin is not always clear.

This is an orange liqueur. You dry-steam orange peels in brandy, add sugar, and that's it! For example, Cointreau uses various types of oranges—sweet and bitter—to create its recipe.

That's a great question. Grand Marnier is also an orange liqueur, but the base spirit is more noble—cognac. In that sense, Grand Marnier is more of a curaçao than a Triple Sec.

We never stop unraveling the knot! Curacao is an orange liqueur (yes, yes, once again) originating from Curaçao island. It is older than Triple Sec, and according to legend, it was this very liqueur that inspired Edouard Cointreau to create his recipe.

Curaçao is blue if you add blue colouring E133 inside. Otherwise it will have the color of its base spirit and/or orange peel. It only exists in Tintin, there are blue oranges, the rest is pure marketing genius.

Any product bearing the mention of Triple Sec. Now that you know a bit more about this product, I think the reason is clear. There are plenty of alternatives, see the recommendations at the end of the article.
If you truly don't have time to go to the supermarket or order online, then use curaçao and Grand Marnier if you have them, for example.
If you're truly feeling down, try orange juice. No, I'm not serious, don't do that.

The same answer, but with a different Triple Sec

A bold attempt by Cointreau to walk on Grand Marnier's grounds, and especially to confuse us. This is a Cointreau with a cognac base.
I am joking, but it is also a logical product following the partnership between Cointreau and Rémy Martin, which makes cognac.