How can I get a pre-release org?

Generic links that will work for all the releases in the attached document.

Timeline Document - https://trailblazers.salesforce.com/0694S000000ftoM

Pre Release Org - sfdc.co/RR-Pre-Release

If you already had an existing pre-release org its now on SUMMER 18. You can log in and check it. As Spring 18 pre-release program is closed(https://www.salesforce.com/form/signup/prerelease-spring18.jsp), you won't be able to create a pre-release org.

Summer 18 pre-release program has not been started yet(Page does not exist) https://www.salesforce.com/form/signup/prerelease-summer18/

Keep an eye on above link to you will be able to create new pre-release org.

I believe you can raise a case to get a pre-release org or wait for your sandbox to get SUMMER18.(https://www.salesforce.com/blog/2018/04/summer-18-sandbox-preview.html) .

Edit: if you stumbled upon this the pre-release org is available

https://www.salesforce.com/form/signup/prerelease-summer20/

or

https://www.salesforce.com/form/signup/prerelease-releaseName/

Pre-Release Sign Up Org for Summer 18 is yet not been Opened. Below screen shot attached with Key Dates for Summer 18