As long as your mortgage is for less than the value of the home you are selling, then you have "equity" which worksas your deposit. So you need to workout how much equity you have: How much mortgage you have paid off + any rise in value of your house (how much you realistically think it will go for).. and work with that as your deposit figure.
If you are in the unfortunate position of not having equity, i.e the price of your house has fallen below what you paid, and below the amount you have paid of your mortgage, then yes, you would need to find deposit money elsewhere.