Cheapest place for Playstation Store/PSN credit?

Try avforums classifieds.

I usually use instant gaming and never had any issues.

instant-gaming.com/en/…rd/ (edited)

I've always used ShopTo unless there are deals called out here

I usually compare things like that on allkeyshop

Shopto is usually my destination too, particularly because their biggest percentage drop is usually on the £40 or £50 cards, which is great for subs and games. There's still a good discount on £20 but it drops off for the smaller denominations.