Prague doesn't have a big gay retail scene, but it does have one proper dedicated store rather than a page of filler.
Gaymegastore
Prague's main gay sex and lifestyle shop, in Letnany in the north-east of the city, stocks underwear, fetish gear, toys and accessories, with helpful staff and a strong online side. It is out of the centre but easy to reach by metro, and the obvious first stop if you are after gay-specific shopping in Prague.
Beyond this, the indoor cruise club Klub Sabinovka in Zizkov also runs a small adult sex shop, and several gay venues around Vinohrady sell basics at the bar. For everything else, the city's general boutiques and department stores around Wenceslas Square are all relaxed and welcoming.