The out-of-print trades on Secret Six, like those of Birds of Prey, are just ridiculously expensive, when you can find them at all. I don’t even have any copies myself, personally. Every once in a while, I will hear of someone getting an amazing bargain, but they seem pretty rare.
Normally I would say Comixology or the DC app is probably your best bet, but my understanding is that DC is reprinting a big collection late this year, that features The Villains United mini, the Villains United oversized special, and the first Secret Six mini, all in one volume, and it’s super reasonably priced (I think it’s just twenty bucks for all those books?).
No one has told me specifically, but it’s a numbered volume, so I am assuming that if it sells well they will cover the rest of the series. The books sold phenomenally well in tpb previously, so cross your fingers!