The lab was so unusual, in part, because illicit fentanyl is so rare. U.S. Drug Enforcement Administration officials say they have documented only three fentanyl labs in the United States in the past 25 years.
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In [1999], coroners in about 40 U.S. metropolitan areas attributed 53 deaths to fentanyl abuse. Deaths from heroin and morphine abuse numbered 4,820.
This last would seem to imply that fentanyl is used only about 1% as much as heroin. Once you count in the fact that most of that 1% are probably people using diverted pharmaceutical fentanyl (patches, Sublimaze, etc).... I think the chances of finding fentanyl or its analogues in heroin now are EXCEEDINGLY small, basically zero.