[Quoth]

I've bought two different 3rd party ones which at the time were the cheapest on Amazon UK, about 1/2 price of original Kobo price, but the Kobo model is about 40% cheaper than at release now. The 3rd party pens are about 1/4 price of official MS Surface Pen. Models with two buttons on barrel, AAAA cell and no BT (only used for One Note on Surface). Shelf life of alkaline AAAA much longer than rechargeable cell (which are not replaceable) and run time is about x4 on the Alkaline AAAA. Rechargeable makes sense for larger or higher consumption things. The rechargeable 3rd party pens are also about twice the price.

Some kinds of Alkaline 9V packs have 6 off almost AAAA cells inside, others use oval layered cells. They are slightly shorter, but if the bonding clip to next cell is left on + end then it fits. Note that ALL Alkaline cells have the seal at the negative end and larger cells have a fake nub at the top to simulate top cap on Zinc-Carbon, so the AAAA often are nearly flat at + end and have a slight nub (the seal) at the - end, thus carefully observe + & - marks on cell and orientation required for the Pen.
Here the AAAA cell is often in a cheap packet with a Lady cell (slightly fatter) and maybe two kinds of 12V batteries for car fobs or doorbells. The Duracell and Energiser are massively overpriced and testing of regular AA size shows them no better than cheap Alkaline cells.

Both perform identically to the Kobo Pens we have. Just look for whatever is cheapest with the two buttons on the barrel and no One Note (Bluetooth) option. I think most Surface models since Surface 2.
I've used Wacom pens (not on Kobo as they are not compatible) with eraser tip at the top and find that slower than an erase button on the barrel, though it mimics a pencil, because you've to turn it over. I've used four Wacom models (one on a Remarkable and one on a Lenovo X201) and find them inferior to the Apple Pencil or the Kobo/MS Pen (MS liked it so much they bought the company that invented that type).

The 3rd party pens tend to come with spare tips. The Kobo didn't and a box of spare tips was expensive. None of mine use the same tips.
The tips are soft plastic that will wear away if you press too hard. The non-EMR Wacom all have hard tips because either for glass or a digitiser tablet.
The original reMarkable has a textured plastic plastic surface and three navigation buttons, the reMarkable 2 has glass as it's actually cheaper and more suited to the Wacom EMR pen, but thus more awkward to write on.

Unless you use the Sketch pad (saved pages appear in My Books as svg files) there is no point to the pen on a Libra 2, except if Kobo add support. Turning it "on" in devmode makes almost no difference and it works (thin, fat, erase) on Sketchpad out of the box. Sage & Elipsa have the Sketchpad app when devmode is on, but it's pointless compared to Basic or Advanced Notebooks.