Apologies for not replying sooner! I purchased my PIAA horns from Amazon UK. Cost £47. Just had a look on Amazon and I see these are marked as currently unavailable. (I did see them recently on DemonTweeks for about the same price). They were simple to fit (I followed a YouTube vid) - No need for a relay as the draw for each horn is only 2.7Amp - the factory horn fuse is 10Amp. Here a link to a US Toyota forum if you're interested in more obs on horn replacement https://www.toyotanation.com/threads/horn-upgrade.1650404/.
Regards , Jack