In short, unfortunately you can’t.
We do offer pre-order on some items, so if you see this option where the 'Add to Basket' button usually is, you can place an order to secure your pair when we restock.
However, if our pre-order option isn't showing, then you'll need to wait for our next restock. Just below the product information each page we have a 'size not in stock?' button - click this and follow the steps to add your details to our waiting list for a notification when your size returns.
Alternatively, you can drop us an email to email@example.com and we'll be able to let you know if and when we have some more due in. We can also add you to our waiting list for a notification as soon as they're back.