Headway makes some smaller cells that would suit you quite well I think. They have a 10Ah, 12Ah, and 15Ah version. I'm not sure where you can get them in Europe, but its worth a look.
Here is a US distributor:
Lithium LiFePO4 Cells - Headway Headquarters LLC | LiFePO4 Cells, LiFePO4 Battery Packs for EV, Lithium Iron Phosphate Battery
The price is quite a bit lower than the options you've posted.
Edit: found a possible source.
http://www.litrade.de/shop/Akkus-Zub...hparam=headway