The very best is the SBS range formerly by Hawker. They are AGM. You pay for quality…
https://uk.farnell.com/enersys/sbs30/battery-lead-acid-26ah-12v/dp/2070725
Robert.
Still expensive, but less than Farnell; yes I can vouch for them, as they were used in some our kit on BT
https://www.tayna.co.uk/enersys/powersafe/sbs30-12/ £273
https://www.batterycharged.co.uk/shop/batteries/vrla-batteries/enersys-powersafe-sbs-30-12v-26ah-battery-1757274.html £212
There’s often some on eBay, but that’s obviously a bit risky on reliability depending on age and previous use.
Alternatively if the full 26Ah isn’t required, these might suffice, either as Odyssey or Harker
https://www.ebay.co.uk/itm/335189347603
https://www.tayna.co.uk/industrial-batteries/odyssey/pc545/
Bill