Typically, these are called SCART switchers. Most any switcher should work fine, including automatic switchers. Towards the cheap end, you have the Bandridge switchers, which come in both manual and automatic variants; and there’s the Otaku Games SCART switcher, which is manual only. On the higher, significantly-more-expensive end, you have the gscartsw and Hydra switchers, which are fully automatic.
The main things to remember are:
1. Make sure that the male-to-male SCART cable that will link the switcher to the OSSC is fully-shielded. You can get cables like this from Retro-Access and Retro Gaming Cables; although cheaper ones from other manufacturers, like Sony or KabelDirekt, should work fine, but, if you notice any interference in the video and/or audio, a cheap cable between the switcher and OSSC may be a contributing factor.
2. If you aren’t going to be connecting the output of your switcher to professional AV equipment, like a PVM/BVM, or Extron or Crestron switchers or video processors, then your cable preference should go: CSync (only if the console natively outputs composite sync), sync-on-luma, then sync-on-composite. Avoid cables with built-in sync strippers; they are more expensive and entirely unnecessary if everything is going into an OSSC.
3. If, instead, you are going to connect consoles to pro AV equipment, you will need to use clean composite sync (not sync-on-luma or sync-on-composite). If you intend to use the sync stripper feature on the gscartsw, you should make sure that your consoles’ SCART cables do not have sync strippers in them. You can encounter issues, like failure to sync, when you have more than one sync stripper in use; so you should pick between having sync strippers in the cables or using the sync stripper in the gscartsw.