Women have been wearing athletic clothing for a very long time.
Here is a Roman wall mosaic from Sicily showing ancient Roman women lifting weights and playing ball. They are wearing bandeau bras and loincloths, probably in linen. From Villa Romana del Casale outside the town of Piazza Armerina, in Central Sicily. Mosaic probably made in the 4th century A.D.
Unless you can get the shirts and material to make them at wholesale cost then it will probably cost less to have them professionally made. You'll have saved on material & the only labor is creating ads & packaging/shipping if that's how you'll be selling them.