The week after Christmas is the busiest/most profitable week of the year for Broadway. Almost all shows black out their discounts during that period, so it's no surprise that Waitress did so.
TodayTix, I believe, pulls the same publicly available discounts that you can find on your own-- I don't think they have their own special discounts. So while you may be lucky with TKTS, I don't think you'll find any advance discounts.
As said, they won't get more expensive, but they will get more limited. Buy them now.
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