Given the relative ‘lack’ of surround information, if used exclusively for movies/TV, I’d recommend a pair of One SLs as being the best, and probably least cost addition.

If you intend to stream music to the PLAYBAR, and use the surrounds in ‘Full’ mode, that equation changes, at least in my mind. 

Thanks. Mostly for watching tv. We have move fishers but they cut out. We also listen to music but I move the move speakers to other rooms for that. shoujd I get a different soundbar and speakers, or get speakers that work with playbar?  I have a sub. 
we just want good sound that doesn’t cut out when watching tv/movies. It’s a med size rectangular den. 

Are ‘fishers’ actually ‘speakers’? If so, Moves can not be used as surround devices, you would need to get a separate pair of surrounds, that are not battery powered. Right now, you likely have the Moves grouped with the soundbar, and not surround speakers, which would delay the signal to the Moves by 75 ms. 

It’s harder than average to get surrounds for the ancient PLAYBAR these days. If you can find them, I’d probably try to get a pair of Sonos One SLs, but any pair of Sonos Ones, SL or not would work. Unfortunately, the replacements that Sonos now sells, the Era 100s won’t ‘bond’ as surrounds with the PLAYBAR, although they could be grouped, as you are now doing with the Moves. 

I suspect the reason your Moves are cutting out while grouped with the PLAYBAR has to do with the density of the signal (more information in a TV stream than in a music stream) and a level of wifi interference . But given the delay between a soundbar’s TV input and a grouped room, it may not be worth it to chase that down, especially if streamed music plays (in sync) just fine. And since true surrounds bond to the PLAYBAR, and not your WiFi, they may not exhibit the same behavior.