Most garden centres and B&Q sell garden lime in 1-5kg boxes.

Wilko £3 for 3kg. Search online for garden lime

It is worth checking if you have an agricultural supplies store in your area. They sell assorted agricultural and horticultural hardware and materials in more modest quantities than the large suppliers. You will probably be able to buy your lime in 25kg and 50kg sacks.

Hi, I agree with Bulbaholic, also try builders merchants usually sell hydrated lime, and it's a lot cheaper than the garden centres, Derek.

Thinking about this, what do you mean by "very acid"? Adding lime isn't necessarily the best way of improving the condition of the soil.