Ready-to-wear
 

In Spanish, I would use (the Gallicism) prêt-à-porter.
Does anyone have a better idea? (I.e., another Spanish term in use?)
The English refers of course to,
1. clothing made in standard sizes; ready-made clothing.
And as an adjective,
2. pertaining to or dealing in such clothing: the ready-to-wear business; a ready-to-wear shop.
(As opposed to made-to-measure, made-to-order.) (In Spanish "a la medida" or "hecho a medida".

A few dictionaries list 'de confección' as the translation for 'ready-to-wear'.

Other translations:

listo para llevar (have a look here (Warning: lots of pictures to download), to see that someone gave it as a translation for prêt-à-porter)
ya hecho
confeccionado

¡Muchas gracias, Señor Speedy ! :)