Muhammad,

There is no need to buy the rather expensive WOM memory the other posters have mentioned. There is a little known but simple trick to convert almost any conventional memory chip to be WOM compatible:

Cut off any pins labelled 'OE' or 'RD'.

If you need bulk quantities this process can be quite labour intensive - unfortunately you cannot buy devices with these pins removed as the American government have classified them, along with some data encryption algorithms, as "too secure for us to crack".