EEMB batteries on amazon are pretty good - Ive used their batteries to replace old ones in bluetooth speakers.
Its easy just to snip the connectors off on the new and old battery. strip some of the plastic back and splice the wire from the new battery together with the old connector. you can either use some solder or heatshrink tubing to hold the cables together, then seal it up using gasket maker from
halfords or silicon bathroom sealant.
I did my 520 the same way a few years ago.The only thing I didnt do is tape the new battery down with double sided tape so my unit has always had a bit of a rattle to it.
I'll probably need to replace it again very soon.
If youre going to go down the DIY route - take the most care when trying to slip something thin underneath the screen/digitiser those things are the most fragile and will break/crack if you put too much force on them when trying to take them out. Im a guitarist so I just used an old guitar pick to get inside the unit.