After Andrew’s comments about the perils of singing along to the recording, here are some rehearsal aids, which are especially useful for those of us who do not have or play a piano. They all have their advantages and disadvantages but all are better than not doing any work at home.
Choraline: https://www.choralineapp.com This rehearsal aid was around a long time before the internet. It is still possible to buy the CDs but there is now an App and the rehearsal material can be downloaded to that. The App is free but you have to pay for the downloads. You just buy your own voice part but the other lines are ghosted in and yours is made prominent. It may be a bit old fashioned but the instructions are very clear and you can start by slowing it down before gradually moving on to full speed. However, the things some like about it are the very things that others dislike, so it’s not for everyone.
Cyberbass: http://www.cyberbass.com is free to use but you have to work online as you cannot download the material. It is possible to purchase downloads in MP3 form from this site.
John Fletcher: https://johnfletchermusic.org/ You can register for a free account which applies to music which is out of copyright. For music still in copyright there is a subscription.
Choralia: http://www.choralia.net Another free site which uses synthesised sound. Files can be downloaded free in MP3 form but voluntary donations are appreciated.
You will not find every piece of music on every site, but this should not be a problem with something like the St John Passion.