https://launchrock.com is the classic tool for this.

Email signups don't really validate your idea anymore, even if they manage to get you some emails.

There was a post a while ago on Reddit where someone built this little project:

https://fauxbuy.com

Basically lets you add a fake buy button easily and emails you when someone attempts to buy.

I'm building something to do exactly this at https://nichetester.com - I should note that I used the method described in the OP to validate it and it worked really well :).

landbot.io is interesting. Try it out