Available as an assembled Module and as a DIY project.
This Module is currently available.
MFOS Euro AR Envelope Generator
Envelope generators are a staple of any modular synth. Having lots of them gives you lots of options when it comes to VCA control, VCF control, delayed modulation, and AR modulation of anything else your heart desires. This dual AR generator is very cool and each EG features triggered, gated and repeat mode operation. When repeat is on (while in triggered mode) the EG continually goes through the attack and release cycles. In triggered mode the front edge of the gate (or a trigger signal) is used to trigger the circuit into operation. The attack cycle begins and completes and then the release cycle starts, the circuit can be re-triggered as soon as the release cycle begins. In gated mode when a gate is applied the attack cycle begins. If the gate is removed before the attack cycle completes (output attains maximum voltage of 10V) the release cycle begins. In gated mode, with repeat on, the EG repeats when the gate is low (when short times are used). In gated mode, with repeat on and the gate is held high the EG will go to and remain at the maximum output voltage (10V) until the gate is released at which time the EG will begin to repeat the AR cycles (when short times are used). Primarily repeat mode is designed to be used in triggered mode.
https://synthcube.com/cart/mfos-en/mfos-euro-ar-env-black-version?limit=100
These merchants probably sell this module. Huh?