With the invention of HD Radio, there hasn't been much interest in adding analog FM subcarriers. Having worked for stations running SCA's, they are kind of a pain in the a$$. But to answer your question:
Many of the broadcast equipment manufacturers built SCA generators. The Modulation Sciences was the preferred, mainly because it came with audio processing/limiting built in. If you don't mind adding a Aphex Compellor or Dominator ahead of it, I would recommend SCA generators from:
Broadcast Electronics, CBS Labs (can also be found rebranded as RCA), or CRL. Just be aware that you might want to replace the electrolytic caps in any of these old SCA generators before putting into service. After all, these boxes were built 30 years ago.
I scanned E-Bay and saw two Modulation Sciences SCA generators. One was listed for $150 and the second for $90. Given the interest in SCA's anymore, you could probably make them an offer for $50 and pick one up.