You can get white hollow doors which cost about 40 Euros. If you want to have a heavier, white door you need "Paint Grade Doors ", which will have an undercoat of white on them, or, you can buy an "unfinished" wooden door.
You can also buy Engineered Doors, which are as good as solid wooden doors. They are cheaper, but make sure you get your sizes right, as they only be planed very slightly. If you want these in white, you will have to get a universal primer first, and then paint them in your colour of choice.
Get plenty of quotes, some places are very expensive, but solid wood doors cost alot anyhow. Woodies have an offer on internal doors at the moment.