Any pair of Sonos speakers if the same type (both Ones, or Fives, or PLAY:3s) that run S2 will work as surround speakers for the Arc. The only speakers that will not work are any S1 speakers, as you’ve found out, or the Sonos Roam or Sonos Move. Those two speaker types are designed for moving from space to space, and don’t have the capability to be bonded as surrounds. 

If you want to save money and buy used, get a used pair of Play:1s or One SLs. Getting a pair of Ones is unnecessary because you cannot use the microphones and voice assistant when the speakers are connected as surrounds.

Here is a full list of surround options:

https://support.sonos.com/s/article/4807?language=en_US

/thanks Bruce, is there a S1 or S2 Play One/ Play One SL or will any examples of these do?

I believe the Sonos One series, much like the PLAY:1 series, can run either S1 or S2 (not at the same time). I use pairs of PLAY:1s for both of my Arc’s surrounds, but Ones would have been fine, too. You can mix and match Ones as needed, but if you’re buying new, for the express purpose of surround use, the One SL is the best deal, same sound, lower cost as it doesn’t have the microphone. GuitarSuperstar’s post covers this. 

In case it wasn’t clear, you do need a pair of S2 speakers.  You cannot setup a single speaker for surround sound, as you were intending to do with the Play:5 gen1.

Thank you for the clarification but I was not intending to use just one Play 5, rather I was going to buy an additional pre-owned example to make a par.

I am now looking at Play Ones’ but wanted to make sure there wasn’t a S1 ad S2 version so that I avoided the same trap!

Many Thanks to all for the information.

I have now purchased a new pair of One SL’s and am awaiting their delivery.

Many Thanks for all of the help and advice.