The shop they visit is maybe FIRST東京アメ横店, but I’m not completely certain. Bazooka.
The cheapest set of three walkie talkies I can find from ten seconds of googling is a little under 6000 yen, so a bit more than what they paid (though Oyaji was surprised to be paid at all). Though these ones don’t actually match the ones he’s holding in the manga, and I have little doubt that the ones he’s holding are a real product.
Heh, I was gonna say that should be スリーコインさっちゃん. So glad afro-guy got there first. Yui’s working on her tsukkomi slap, I see.