Well, as you asked:
If you won't tell, I'll admit that I'm finding the books in the usual places (Please keep it on the QT, as I've already read enough forum threads to know Barbara's opinion of libraries and 2nd-hand shops!), and I am grateful to those resources for preserving her works until I became aware of them.
I found out about Barbara last week via the Avon and Holland page, and immediately interlibrary loan requested the Marian Larch series. I picked up YHTRTRS, FP and FFM from the library on Saturday, and (fortuitously!) bought TAT while visiting a library book sale later that day (it was the only book they had by Barbara, or I'd have purchased more).
Then I had myself a little Marian Larch festival from Sat evening to Monday morning. ;-)
I've since been searching out (all) Barbara's other books so that I can enjoy those, too. The toughest one to obtain may turn out to be "Clean Sweep" in "A New York State of Crime".
Meanwhile, I am re-reading the ones I have, and really savoring them. (Though, I admit that if one reads them all at once, one notices little continuity glitches -- or at least, questions. FWIW.)
P.S. - My other favorite mystery authors include (but are not limited to): G.Heyer, D.L.Sayers, P.D.James.