Yamaha - FC 5 (Sustain Pedal) 29 €
For electronic keyboards that have an input socket labeled “SUSTAIN”, the FC5 is used in the same way as the sustain pedal on an acoustic piano. A specially designed rubber sole sticks to the ground and stays there while you play. The FC5 can also be used as a foot switch controller for synthesizers, sound modules and drum computers. The functions depend on the device to which you connect it using its 1.2 m long cable with a mono jack plug.

Boss FS-5U (foot switch) 35 €
The Boss FS-5U Foot Switch is a foot switch that applies an effect for as long as you hold the pedal down. So it is a momentary unlatched type switch. You connect the switch with a 6.3mm TS jack cable, to use for various audiovisual applications.

Moog - EP-3 (Expression Pedal ) 49 €
Rugged Expression Pedal for Your Pedals and Synths
The EP-3 is a robust and reliable universal expression pedal designed to work with any keyboard, synthesizer, or pedal with an expression or CV input. It features a newly designed cam system with extremely smooth performance and durability, a polarity switch that ensures compatibility with non-Moog products, a scalable output knob for specifying musical range, and a ¼” output jack on the front panel that allows you to use the supplied 6.5’ cable, or select a different cable length that perfectly suits your needs.

 

For people who do MIDI I can wholeheartedly recommend Behringer FCB1010: https://www.thomann.de/intl/behringer_fcb_1010.htm

A very versatile tool for triggering DAWs and whatnot, I use mine in Ableton to select tracks/clips, record and of course for the Looper. Also nice to have two expression pedals for FX automation/sends when playing.

It can also be coaxed to switch channels on guitar amps, but it does not send CV.

 

The first two are the same: the simplest non-latching footswitch. You can get them even cheaper.
But you can also get one that looks and feels like an actual piano sustain pedal.
I find the standard ones fine, and the throw is shorter so I can use it quicker.

The third one is more important that it's a good one, as it scrolls through for volume/swell on a Hammond, or as wah-wah, or for modulation, vibrato, or anything.
The M-Audio are cheaper when you need to buy more than one and you're gigging a lot (wear and tear) but the Moog will be a better make.