The battery on a Kobo Forma has a black and a red wire which are soldered to the motherboard. You can find a guide on iFixIt.com (
Kobo Forma Teardown ). I've attached an image from there showing the battery connections. Please note that the comment about damaging the battery while removing it from the double sided tape holding in in place is very valid. Even using dental floss and running it under the battery, you still have a decent chance of damaging the battery. The first image allows you to read the battery part number (PT158098) though PR-158098N and 1ICP2/80/98 are also the same battery (1.5mm x 80mm x 98mm, 1200mAH).
You can find several prices for a PR-158098 on Amazon UK. The cheapest option I saw was for a
Limtula Universally PR-158098N 1200mAh .
The two I purchased had a two pin connector which I cut off since the Forma solders the battery to the motherboard.