'89 YJ - Source for OEM leaf springs

I don't know. If you can get one that looks good, there are pros and cons to using the entire spring, or just one leaf.

Some break-in occurs. If you mix and match, you are likely to accelerate the process of spring wear. Plus you will need to install a new center bolt, which can be a pain in the neck. Make sure to use the proper grade bolt if you do. On the other hand, I think you are right, in that the four or five bottom leaves from your broken spring, are likely to be a good match to the other side. Don't forget to use new U-bolts, too. Maybe there is a pair from a not-too-old vehicle somewhere close to you.

Earle

Around here there are lots of old YJ springs kicking around. They all go flat so folks toss them and get lift springs. You should be able to find a leaf fairly easily if you check with your local 4x4 clubs.

I know the big rig truck spring shop that replaced my CJ springs can order just one leaf in.

Mike

Welding changes temper of the spring in that area, which would lead to cracks (if tempered to hard) or a nice soft spot bend if the temper is too soft. Basically impossible to get it just right... You might luck out but I'm sure the odds are seriously against it.