Tremolo pedals are a bit more distinct than chorus, phase and flange effects. However they're still a form of modulation. It's modulation of volume moving it up and down at a certain speed, and following a certain wave form (usually square). Certain pedals let you change the wave form, others bring other modulation into the mix, and some even offer double tremolo effects.
Tremolo Pedals
£369.00
Source Audio Source Audio ZIO Analog Front End + Boost
£205.00
Joyo Joyo Tremolo JF-09
£28.99
Electro Harmonix Electro Harmonix Lester G
£199.00
£125.00