If your clients are self funding there is no need to do anything special before setting up a client contract, as the Self-funded option appears in the list of available purchasers automatically as shown below.
Similarly, if the customer's care is paid for by someone who is set up as a contact of that customer (on the People tab) they will also appear as an option in the Purchaser select.
In the case that a customer is funded by government (or some other funder of multiple recipients) you will need to set up that organisation as a Purchaser by selecting Purchaser from the New menu
When you do that the record that you set up has the Social Purchaser field (on the invoicing tab) checked which means it will be available on the Purchaser select list for all customers.