Be specific about what information you want and how you would like to receive it. Quite a range of organisations do now produce standard information in large print, braille or tape formats - and some will produce personal correspondence in these formats, on request. It is always worth asking for information - many organisations are keen to respond to their customers' needs. Even if the answer is no, you have made the first important step by making the organisation realise that there is a demand. What is more, with modern computer equipment, producing large print documents costs either very little or no extra money at all.
Braille is relatively straightforward to produce.
RNIB has detailed information on organisations which offer braille transcription services - including RNIB itself. Address your complaint to someone specific, either to a customer services officer or to the most senior person in the organisation. Make sure you get their name, title and address right. Keep a copy of your letter. If you are unable to write, telephone the organisation concerned and tell them that you are unable to send a letter because you cannot see well enough to do so.