Current hybird battery packs are designed the last up to 200,000 miles and possibly longer. Manufacturers are offering 100,000/8 year warrantys to give consumers confidence.
There's little known about buying replacements as, according to Toyota, they have never replaced a single battery. This would suggest that in the event of them failing, they're not going to be cheap!
In tmers of disposing of batteries, Hybrids use NiMH batteries which are benign and can be recycled. Toyota and Honda both offer to recycle dead batteries with no risk to the environment. Toyota also offer a fee for each battery so that the incentive is there to recycle the batteries.