The brand appears on all kinds of items from shirts to footballs. It is also another one of the things Oda puts in his manga for fans to spot (much like Pandaman). The most commonly seen Doskoi Panda logo can be found on Sanji's pink apron.
According to Oda, Doskoi is a popular brand-name franchise in East Blue and its headquarters is on Mirrorball Island (which was featured in the Jango's Dance Carnival featurette alongside the One Piece movie Clockwork Island Adventure.) It is a very high-end brand and as such, it is very expensive. One shirt might cost a person over 10,000.
Oda later in the series also gave the name of another fake brand, Cyberpanda. Although fake, it is far more expensive than the original Doskoi brand.