Have you considered getting a radio trigger system? Phottix is good. Pocket Wizard too. It is extra money, but it works better than the old flash-triggered system, and over a lot longer range.
Are these Canon brand speedlites or are they 3rd party flashes?
If these are Canon brand speedlites then you can use a Canon 90EX on-camera flash which is capable of operating as an on-camera "master". It's not a particularly powerful flash but if you're just looking for something to replace the pop-up flash you had on your last camera, it is able to perform that role.
The Canon 600EX-RT (Canon's flagship flash) is able to perform any role (optical or radio... on camera master or off-camera slave.)
I have a couple of 600EX-RTs and a Canon ST-E3-RT (on-camera radio trigger) and I love the reliability of the system.
I've had really great experiences with PocketWizard Plus III units.
If you're trying to work on a budget, there are a lot of aftermarket flashes with optical slave functions. As long as you're the only flash shooter there, and you're not using multiple preflashes, and you don't mind manual everything, they get the job done.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM