Organic ericaceous compost

I am something of a novice gardener so please bear with me....!

I have 2 lovely blueberry plants that have just arrived through the post. I know they need an acidic soil and my garden soil is slightly too alkaline (pH6.5-7). I am planning to keep the blueberry plants in large containers with an ericaceous compost. I have a strong preference for organic products wherever possible, but I am finding it difficult to find an organic ericaceous compost that isn't going to cost a small fortune! 

Vital Earth do an organic ericaceous compost but the only company that has the large 60L bags in stock are going to charge nearly £30 delivery!

If anyone reading this has another suggestion I'd be very happy to hear about it! Whilst I have a preference for organic I would willingly go for products that are produced to organic standards.

Thanks!!