Broccoli, Brussels sprouts, Cauliflower

Extracts of 3-day-old broccoli sprouts (containing either glucoraphanin or sulforaphane as the principal enzyme inducer) were highly effective in reducing the incidence, multiplicity, and rate of development of mammary tumors in dimethylbenz(a)anthracene-treated rats. Notably, sprouts of many broccoli cultivars contain negligible quantities of indole glucosinolates, which predominate in the mature vegetable and may give rise to degradation products (e.g., indole-3-carbinol) that can enhance tumorigenesis. Hence, small quantities of crucifer sprouts may protect against the risk of cancer as effectively as much larger quantities of mature vegetables of the same variety.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC23369/

https://everythingiknowaboutthatilearnedfrommysleddogs.wordpress.com/2017/03/12/third-brother-angus/

For example, a commercial product:

Supplement Facts
Serving Size 1 vegetarian capsule
Amount Per Serving
Broccoli super concentrate extract (seed and plant) [providing glucosinolates] 400 mg
I3C (indole-3-carbinol) 80 mg
Watercress 4:1 extract (whole herb) 50 mg
Rosemary extract (leaf) [std. to 20% diterpenic compounds, providing carnosic acid/carnosol] 50 mg
Cat’s claw extract (bark) 50 mg
Apigenin 25 mg
Cabbage extract (leaf) 25 mg
DIM (3,3’-diindolylmethane) 14 mg
Other ingredients: vegetable cellulose (capsule), maltodextrin, vegetable stearate, silica.
Non-GMO

http://www.lifeextension.com/Vitamins-Supplements/item01468/Triple-Action-Cruciferous-Vegetable-Extract