I bought one of those cheap clip-on JOYO tuners from Ebay a year or so ago, and now it needs a new battery.
I popped in my local Tesco only to find a pack of 8 was £7, time to order a new tuner me thinks.
While in IKEA tonight I stumble across a pack of the required batteries, CR2032, 8 for 95p
I guess the batteries will last longer than the tuner.