According to the blog of a headphones shop, AKG K701 was sold out for the first time since the opening of the shop. AKG K701 is a high class headphones and priced 74,900 yen (approx. $750). Mio wore the headphone in the episode 5 of "K-ON!".

Screenshot of the scene

Source: E-Earphone blog

Mio used a mobile phone NTT Docomo FOMA D902i in the same episode. It's an old model released in 2005 but was also sold out in the used mobile phone store.

Screenshot of the scene

Sources:
Senaka Blog
GIGAZINE

Update========================
Mio's lefty base guitar "Fender JB62/LH/3TS" was also sold out in 2-3 weeks and Fender Japan sent an order to the factory to produce the guitar in the amount of two-year sales.

Source: Ascii