This site seems to be selling a new battery with a case and contacts compatible with the Macintosh Portable.

Click to expand...

Upon further investigation, that company wants you to send in your old battery which they will rebuild for you. They claim they seal up the casing after they are done, which is a process I wouldn't yet know how to do. That you have to send them an old battery makes that option much less attractive to me as I would have to buy an old Apple battery in addition to their fee. In addition, I would have more fun doing the rebuild myself.