I have never used a specialist tango music supplier: I haven't found the need.
My most frequent source has been the vendors (mostly, but not exclusively, American) that trade via Amazon's Marketplace. I was surprised to find just how wide is the range of tango music from all of the main labels listed on Amazon in the first place, and then amazed at how cheaply a lot of material is sold by these additional sellers. Only today, a disc of instrumentals by OTV has arrived in the post (in the RGS Tango Collection series). It cost me just £2.79 + £1.26 P&P. Amazon UK, itself, would have charged me £9.79 or £7.49 as an MP3 download.
I have bought dozens of discs this way, and rarely paid more than £5.
I buy fewer and fewer CDs, though, and download more and more. I have noticed that Google's recently launched Play is usually slightly cheaper than Amazon (who is usually slightly cheaper than iTunes). Google MP3s are usually 320kbs too. Another thing about their pricing policy is that double CD sets are often the same price as single ones. The excellent Lantower Grandes del Tango series is a good example, and they are typically £6.49 for the equivalent of 2 CDs of material.
I have no commercial connection with any music seller, and so only name these because they have allowed me to build a really quite large collection of music (some of it quite obscure) for very little money.