As a general rule, if you stat up a 10th-level PC as a monster, you get a CR 5 statblock, so that's about the criteria you should be looking for. Exact level equivalents will be impossible to find in anything published since about Candlekeep Mysteries, but the Rivals from Call of the Netherdeep give a good rule of thumb for how to implement PC equivalents under the current design paradigm.For specific examples:
Sorcerer: Prismari mages (CR 2, 4 or 7, Strixhaven), Kelek (CR 5, WbtW), Occult Extolant (CR 6, CotN), Occult Silvertonge (CR 8, CotN)
Monk: NPCs from the Immortal Lotus (CR 5+, though I find these calculations wrong, CM), all the 'Monastic' NPCs (CR 5+, CotN) and of course you can easily reskin any githzerai!
Warlock: Oriq Recruiter and Bloodmage (CR 4 and 9, Strixhaven), the Yuan-ti warlocks (CR 4 or 5, MPMotM), Warlock of the Great Old One or Fiend (CR 6 and 7, MPMotM), Githyanki Starseer (CR 7, Spelljammer).
And that's not including just taking the Mage statblock and switching its spells and ability scores around a bit.
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