There are very few third-party suppliers. eSoundz / Sonic Reality may have some that are of interest.
Or, you could try using the SampleTank Editor (in your User Area) to build your very own instruments.
https://www.youtube.com/watch?v=VxaiocQrjjQST4 Editor User Manual wrote:. Prepare your samples in the proper format. They must be AIF or WAV files, 16 or 24-bit, and at a sample rate of 22050, 24000, 32000, 44100, 48000, 88200 or 96000 samples per second. Samples may be looped with a single loop start marker and the loop end marker set the same as the end of the sample (note that loop end markers placed before the end of the sample will not play back correctly). Unlooped samples must fade to silence.
2. Check the file names of your samples. Sample file names should ideally follow the SampleTank 4 naming convention:
For pitched instruments and drums: A1_v120_w1_e1_rr1 Epic Grand Piano.wav
For loops: A1_v120_w1_e1_rr1_4bar_120bpm EDM Synth Loop.wav