Where can I get a replacement battery for my 2015 MacBook Pro 17"

the last 17-in display in a MacBook Pro shipped in the 2011 model, then was discontinued.

Perhaps you are reading the Processor spec, Intel i7, as the display size?

what is the diagonal measurement of your display? or what is the model number silkscreened on the bottom in gray ink? or what does it say when you choose  menu > "About this Mac"?

The problem with that Battery swap is the old battery is glued in place. The standard upgrade procedure requires you to remove absolutely every component that could be damaged by spills of the noxious glue solvent needed to get the old battery out.

if you would consider a new Mac, that may be a FAR better way to proceed.

After you transfer everything to your new computer and confirmed everything is good, then you should prepare the old laptop for sale/recycling by following the instructions in the following Apple article:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Of course you can still continue to use the laptop by using the power adapter.....at least until the battery swells or fails to such an extent it could prevent the laptop from powering on. I would highly recommend enabling Filevault on the system to encrypt the drive to protect any personal data stored on the internal SSD so if the laptop ever does fail completely you don't have to worry about the personal data stuck on the internal SSD. The 2015 model was one of the last Apple laptops with a removable internal SSD so you could remove it & physically destroy it, or you could install it into an external OWC Envoy Pro Enclosure to use it with your new Mac assuming the old SSD was of significant size to warrant the price of the enclosure (the SSD itself is not that fast compared with SSDs today since it has a maximum transfer rate of about 500MB/s).

FYI, good decision on not trying to replace the battery yourself. The battery in the 15" model requires removing all internal components in order to replace the battery. OWC even recommends professional installation.

Apple appears to have stopped supplying batteries for that model.

the most reliable source is OWC at macsales.com. if not in the US, their products may be available through other resellers locally.