* Stacy - This is the closest you can get phonetically, but it won't be recognizable as a French name.
* Stéphanie - A French name that sounds similar to Stacy and is fairly common.
* Stasie - A less common French name that sounds similar to Stacy.
* Stacey - You could use this spelling, which is sometimes used in French.
The best choice depends on the context and your preference. If you're just trying to say "Stacy" in French, the phonetic spelling is probably the best option. If you're trying to find a French name that sounds similar, Stéphanie is a good choice.