My first stop when I need a score is Luck's Music (lucksmusic.com). They carry all the critical editions of Mahler. For some things, Boosey,com is quite helpful. Also Haus Doblinger in Vienna is really great, doblinger.at and the site has an English side to it. For a lot of Russian stuff I go to sikorski.de and pay through the nose.

But the best source for really hard to get and rare repertoire is musikmph.de with their extensive Repertoire Explorer series. Prices are very, very user friendly and the quality of their scores is excellent.

 

sheetmusicplus.com has a broad selection but they are a retailer, not a publisher.

 

In addition to those already mentioned, you can try these two:

Breitkopf

Universal Edition

 

An example of what I'm looking for: all Mahler's symphonies, Bartok's Piano Concerto no. 1-3, and Janacek compositions.

+1 to the post from @prlj
Universal Edition for the Janáček, Mahler and Bartok. Their Study Score Series is excellent.
Sheetmusicplus.com carries the UE catalog as well (although sometimes the shipping takes a bit longer, depending on your location.)

 

An example of what I'm looking for: all Mahler's symphonies, Bartok's Piano Concerto no. 1-3, and Janacek compositions.

Go to IMSLP.org, find your score (all the works you mentioned are in the public domain worldwide), print them, and have them bound (or do it yourself if you've got the equipment).
Alternatively, have them in pdf format on your tablet or laptop, will cost you nothing and saves shelf space.

Ideally, I prefer basic editions, in paperback format for study purposes both at desk and at my piano. I am only interested in Full orchestral scores and not any piano reductions or rearrangement of any sort.

A paperback study score, specially one with glued spines is far from ideal for piano use, won't stay open and if you stress it too much, the spine will break.
Even more reason to use wire or comb binding for your home-printed scores. As an organist I printed and bound hundreds of booklets using comb binding, and I prefer their handling at the keyboard to most shop-bought scores.

 

Go to IMSLP.org, find your score (all the works you mentioned are in the public domain worldwide), print them, and have them bound (or do it yourself if you've got the equipment).
Alternatively, have them in pdf format on your tablet or laptop, will cost you nothing and saves shelf space.

I mean, yeah...IMSLP is the sensible, correct answer for all of this. No question. But there is NOTHING like a nice bookcase full of well-thumbed scores.

And have you ever held a copy of the urtext of Beethoven 9 in your hands from Breitkopf? Or Mahler 3? It's like holding pure sex in your hands - underscoring the timeless work printed therein...

 

Yes the Urtexts are often not public domain. This is a good point. I could go into specifics but it may be worth paying the premium for the extra quality of printing and binding, scholarship, and owning the score on your shelf; it is up to your tastes and your study habits. Also some publishers sell facsimiles of the autograph scores, which are usually super expensive (printed in color) but I know some people like it.

 

Yes the Urtexts are often not public domain.

Here in the EU, Urtext editions become public domain 25 years after the initial edition. So there are lots and lots of scores in relatively modern, critical editions from the 80's and 90's available. It's not all old error-ridden junk. Mahler comes in the corrected International Mahler Gesellschaft editions from the 60's and 70's that are still used worldwide today (since the new critical edition isn't finished yet).
I don't know how this works in the US with its labyrinthal copyright though.

 

You get what you pay for. Dover scores are a good example: some of them are just fine and have been the standard performing editions for generations. They are not urtext. The Beethoven, Brahms, Schumann, Tchaikovsky, Ravel for example are just excellent. There are problems: any Stravinsky is woefully out of date. The Dvorak 9th is fine, but the 8th it's with is a dreadful edition with many errors. The entire Mahler is questionable; the 7th and 9th are particularly bad - hundreds of errors. The same problem with IMSLP. For casual use and score reading by a non-pro, these sources are ok, but for a professional, a bound and printed score are the way to go, expensive as they can be. If you're into miniature scores, Eulenberg and Zen-on are worth the price,