Any balls from the good companies that are 2 stars or training balls would be good. I would just look for the best deal on training balls. Double Fish, DHS, Joola, Stiga, Yasaka, Donic, Nittaku all have good training balls.
I have heard people who like the Gambler training balls. I don't like them. They seem light and like they break too easily to me. The "3 Stars" feel more like 1 Stars to me. But for training it does not matter so much. GoGo balls are sort of cheap as well, again, the ones that say "3 Star" are more like 1 Stars, but the last time I bought, it was $29.00 USD for 144 so not a bad price. And good enough for training even though they are not as good as the brands above. The GoGo balls last longer than the Gamblers, but they are not great.
I truthfully like Double Fish and DHS for training balls better than anything else but the names in the first list are all great balls for training.