You may find a dedicated sweatshirt/tracksuit fabric section in your fabric store of choice. If you don't, just head for the knitwear section and look for a thicker comfortable knit fabric.
As far as fibers go, you can go all cotton, or opt for a poly or acryl cotton blend.
Make sure to pay attention to the inside of the fabric. Sweatshirt fabrics come in loopback and brushed back. If you want that OMG-it-feels-so-soft-on-my-skin feeling, go for brushed.
Vous n'avez pas à utiliser du tissu sweat. D'autres options sont disponibles, de la néoprène à la mode (aka le scuba) à un choix plus poilu comme le velour stretch (aka crushed velour en anglais).
