It is not easy replacing the battery yourself. I have seen numerous posts on these forums from users having accidentally damaged their laptop while replacing the glued in battery. Also, keep in mind you must store the removed battery safely and have it properly recycled....it cannot be put in the trash as is has toxic materials and can be a fire hazard if the battery is punctured or crushed.
I agree OWC is the best place to get a third party replacement battery, but iFixIt is another reputable vendor.
You may want to check whether your laptop is part of the Battery Recall for that model....only a small number of laptops qualify:
15-inch MacBook Pro Battery Recall Program - Apple Support
FYI, here is the current list of the publicly acknowledged free repair programs for various Apple products (some products may have multiple programs):
Apple Service Programs - Apple Support
As the others have already mentioned, you must be extremely careful with third party Lithium-ion batteries. The quality is extremely poor. I personally will go for the official OEM battery whenever possible, before looking at third party batteries. It is very hard to find reputable vendors selling a quality Lithium-ion battery...the two vendors mentioned are your best options. And reviews & ratings on Amazon cannot be trusted...many have been paid reviews and some ratings are based on older items (perhaps even a completely different product since Amazon does not clear the ratings history when another product takes the slot of a previous item). Plus you do get what you pay for especially in the tech & electronics area.