It will take quite a bit of research to pin this one down, because searches such as: "incarceration statistics" women "anti drug" (0 hits) and "incarceration statistics" women conspiracy (4 hits) in usa.gov do not seem at all promising. usa.gov includes MANY state government as well as federal government reports, including congressional hearings, corrections department reports, state and federal government studies, etc. The search: "prison statistics" women drugs gets 29 hits, and might possibly lead you at least to agencies or people who are interested in the general subject of women prisoners.
The Department of Justice Bureau of Justice Statistics has many pages of information, but I find that its imprisonment statistics do not break crimes down nearly to the detail required here. At best you may find broad categories such as "Drugs" "Drug Offenses"; maybe "narcotics trafficking" or "drug trafficking".
I think that to find this specific statistic - even if there is a report in the Bureau of Justice Statistics that just happens to mention these yearly figures for women imprisoned for this one offense - it will be best to try first in a commercial database on-site at an academic library, where you can search the full text of journal articles. For example, Academic Search Premier gets 16 articles for the TX (All Text) search: women and incarcerat* and conspiracy and Anti Drug Abuse. Quite possibly some of the cited references in these articles will lead you closer to studies that might have been done.