Check out Passion Music at http://www.passiondiscs.co.uk/ . They are a British based firm who specialise in imports from the Balkans and carry a huge stock.

I'd definitely start with the Topic Bulgarian CD. It's an excellent sampler of the country's musical traditions in general. Indeed, I'd take a look at Topic's entire World Music catalogue. Some very interesting stuff there.

Two very good regional anthologies you should know about are:
Anthologie de la Musique Bulgare Vos 1 - 5 on the Chante du Monde label.

Beyond the Mystery; Village Music of Bulgaria. Vols 1 to 3 on the BMA label BMA 1001 - 1003. The Passion Music magazine is currently running an interview with Yves Moreau, who collected the material for the series. http://www.passiondiscs.co.uk/articles/yves_moreau.htm

The Chante du Monde was released many years ago and could well have been deleted by now, but the BMA is pretty new and is carried by Passion Music.

Nonesuch released a couple of scorching LPs of Bulgarian music in the 1970s. These may well have been condensed onto a single CD.

Saydisc have a small but interesting ethnic catalogue and their Vocal Traditions of Bulgaria Saydisc CD-SDL 396 is worth anyone's money.

Finally, the state record company, Balkanton, used to carry quite a reasonable catalogue which was well worth looking into. I don't know if they're still in business and the only link I was able to find for them wasn't working. www.balkanfolk.com/balkanton-folk-cds.php .

Well, not quite finally. The Bulgarian women singers you mentioned might possibly have been the Bisserov singers;

Pirin Wedding and Ritual Songs. Pan. 7005 CD.

And of course it's important to rememember that Bulgaria is just one tiny part of eastern Europe. There's also the former Yugoslavia, Albania, Greece, Romania, Hungary Ukraine, Poland, the Czech and Slovak republics. There's a lot of cracking stuff out there.