just had a quick look on ebay..

I still own an Alesis Nano Synth General midi module somewhere boxed up with all my old kit..
It was a very popular choice way back...

Here's an example of one starting at 25 quid...

https://www.ebay.co.uk/itm/Alesis-NanoSynth-Sound-Module-64-Voice-Multitimbral-Synthesizer-inc-PSU/123727768166?epid=1978116194&

Old midi gear is cool again...