My husband is self-employed, does anyone know what I can provide as proof of his employment?
Are we talking about someone in the UK who is self employed? Is he working for a sole proprietership or does he have a Limited company of his own or a Umbrella company. Either way, he should probablly have a chartered accountant that prepares his information for Inland Revenue. Paperwork around that are should be good enough.
If he is self employed in the US, his most recent IRS tax return should be suffecient and he should have 1099's and the like that prove how he makes money and how much he makes.
A little more context as to what you are attempting to prove specifically and why might help us make better suggestions.