Training in bare feet is so much better. This depends on the circumstances and environment that you are practicing in.
On the grass, Sandy beach or in the comfort of your home, bare feet is practical and very beneficial for building up strength in the feet. Feeling more connected to the ground, the feel of grass or sand between the toes is a nice experience.
On concrete or rough floor I prefer to train in wide fitting pumps, I avoid narrow fitting trainers and running shoes with a spongy sole. Flat soles are proven to relieve back pressure and improve posture over time.
Shoes for tai chi are inexpensive, I think I brought these off Amazon for about fifteen pounds. You can get pumps from the supermarket or local shoe shop.
The important thing is to feel comfortable and enjoy your tai chi