Mojotone 9AB2C1B 17" Reverb Tank
3-Spring Medium Decay
Fits most traditional Fender reverb loaded amps (like Deluxe, Twin, Pro, Super, Vibrolux, etc.)
The 'Medium' decay isn't quite as "surfy" as the 'Long' decay tank that's stock in Fender reverb amps, so it's a popular plug-and-play mod for folks who don't need that huge, splashy reverb sound (I don't know who those folks are, but they must exist). The medium decay makes the reverb more controllable.
$25 shipped in the USA
Here's the stock image--mine looks the same with just a bit of tone-enhancing dust on it.