In Spring/ early Summer if you can find an adult in good condition, (rare).

With a young one - any time except just after a short hibernation.

However, WHERE you buy is far more important. Don't buy from a pet dealer (shop/Centre) The term "Centre" is just a flashy name for a pet dealer.

Click on "Buying a Tortoise" for more info & a list of TPG breeders.

Regards,

Tom

You also need to be very careful which species you buy, as many are not suitable for novices.