The battery in my Garmin Forerunner 305 got to the point where it wouldn't hold a charge long enough for a 6 mile run. A Google search at first didn't result in much, but finally I came across a couple pages that looked promising. One guy used a phone battery with success, but I don't have any phone batteries lying around. Another site I came across indicated that a replacement battery for an iPod Mini is the same voltage (3.7V), about twice the capacity (1300mAh instead of 750mAh), and very similar in size. I bought an iPod Mini battery on eBay for $4.21 including shipping. A couple days later it arrived and I went to open my Garmin. I had read that sometimes the watch pops open of its own accord because the battery swelled, but even if it doesn't you should be able to open it by prying with your fingernails.
I went to pry mine open and realized it was already swelled open, so I just opened it the rest of the way with my fingers.
And Kim tried it out on her run this morning. It's working well!