Is it safe to buy MacBook Pro batteries on eBay?
Another option for longer term (after you have determined why the battery is expanding like it is) may be to see about getting one of the Newer Tech (or other known manufacturer) batteries for your MBP, as they have longer life ratings for the charge. Newer Tech is available from Other World Computing (www.macsales.com), and I've seen it from a few other online sites as well. (I do not work for OWC, but have been very pleased with their products and service) The NewerTech ones I've checked on are rated at higher mA ratings than the original Apple batteries.