There are tons of places online that will rebuild your battery pack.
Battery House | Tech Centre
rebuild rechargeable batteries
You should be aware that for a 25 to 50 cents more per cell (each cell is 1.2 volts, so if you have a 14.4 volt battery pack, it contains 12 cells) you can have your battery pack rebuilt with higher mAh rated cells. So, for example, if your original tool came with 1500 milliAmphour cells, and you had it rebuilt with 2200 mAh cells for an extra $6.00 per battery pack, you'd typically expect to get 47 percent more working time out of that battery pack per full charge. If you have two battery packs, that gives you 50 percent more charging time on each pack while you're using the other.
This web site gives prices on rebuilt tool battery packs for battery packs from 6 to 24 volts rebuilt with 1500, 2200 and 3300 mAh cells:
Rebuilt Power Tool Battery Options at Microbattery.com
And, you should be aware that if you look up "Batteries" in your Yellow Pages phone directory, there will ALMOST CERTAINLY be at least one battery supplier in your area that also rebuilds batteries.
When rebuilding a battery, it's best if the company doing the work uses a hobby spot welder to "tab" the batteries. Tabbing the batteries together means connecting them all in series with short metal strips. Batteries with solder tabs on them are intended for sale to people who rebuild their own battery packs. If a company is making a business out of rebuilding batteries, they should be using a hobby spot welder. It's not a cheap tool, so it shows the company is committed to the business they're in.
I have a Porter Cable MagnaQuench 12 volt cordless drill I bought back in the early 1990's. I've only got one battery pack for it, but I've had the cells replaced in that battery pack at least 3 times now.
GoSunny: Rebuilt battery packs are as good or better than those offered by the tool manufacturer. I've found that tool manufacturers don't use high capacity cells in their battery packs, and I really don't know the reason why not. I'd be speculating if I said that maybe higher capacity cells don't generally last quite as long as the lower capacity cells, and the tool manufacturer is MOST concerned that his tools will get a reputation for burning out battery packs quickly, and that would screw up his income from tool sales way more than a reputation that his battery packs need to be recharged often. At 60 to 120 bucks a pop for a new battery pack, most people would prefer to recharge more often than replace more often. It could also be that tool manufacturers save money where they can, and people don't really look at the milliAmphour rating of the cells in the battery pack when comparing cordless drills. So, that's a place the tool manufacturer can shave $5 off the cost of manufacture without losing any sales.