I can't recall when I last bought a battery from a high street retailer, or a main dealer come to that. Online prices are so much more competitive, plus the convenience of delivery to your door. I generally check eBay but the main battery suppliers are competitive so I've never gone to eBay. The key is to find the battery size code and Ah rating you want, then just Google it for the best prices.
Especially for a rare car like the 126, just looking for "a Fiat 126 battery" is going to get you a very limited choice and higher prices. Asking for "a size 077 [or whatever] battery" will bring in all the suppliers who omitted to include the 126 in their compatibility lists.
I believe 077 is the code for the 126, it may vary between variants. Size and codes chart here:
http://www.puretyre.co.uk/car-battery-specification-chart/