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For 20mm (which is significantly larger than 15mm just to be sure of that)and if you can get them, GW's Dark Future miniatures are difficult to get and not cheap but simply incredibly well sculpted!
As an example the 1/72 Pegasus Gladiators set is really good for that mad max raider vibe.My gladiator faction are all using these models.There's also the Italeri 1/72 Indian Warriors box that I've seen several people use as the basis of warboys too. Check