Is there a way to get a verification code for my iCloud account after my iPhone was stolen?

You can log into www.icloud.com/find without needing a two-factor authentication code.

Also see the following Apple articles for information about lost or stolen iPhones or iPads: If your iPhone or iPad is lost or stolen - Apple Support and

Find your lost Apple device or AirTag with Find My - Apple Support

To log in to other areas of iCloud.com where you do need a two-factor code, see the following from Two-factor authentication for Apple ID - Apple Support

If you don't have a trusted device with you

If you're trying to sign in and don't have a trusted device with you that can display verification codes, you can tap Didn't Get a Code on the sign-in screen and choose to send a code to one of your trusted phone numbers. This text message might include an additional domain validation line that includes the @ symbol, the website name, and your code (for example, @icloud.com #123456 %apple.com). Or you can get a code directly from Settings on a trusted device.

Learn how to get a verification code

You would need a phone number that is accessible from somewhere other than the phone that is lost.

Two-factor authentication can use phone numbers from Androids, landlines, friends' or relatives' phones, but it would have to have been set up while you had access to the Apple ID.

If you don';t have access to any trusted devices or trusted phone numbers, see the following from Get a verification code and sign in with two-factor authentication - Apple Support

If you still can't sign in

If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.