Credits goes to Curdock for telling me the Heroin locations..
You may need to zoom in to see the marked spots
Green= weed spot
Red = heroin spot
HEROIN SPOTS:-
San fierro ship, keep going down till you find a can
Palsides north-west San fierro, Behind the last house, keep going back till you fall down, their is NO CAN and you must find the exact spot to order heroin
Bayside, near to the weed spot, the can is hidden in garbage
Abandon airport, go total north till you find a house, the can is in front of the house
North-west LV, Go inside the warehouse marked on map, go to the end, the can is inside a crate
East Los santos, its near the car wash, under the bridge in a small alley
If anything else needed to be added i will try to add em, i know the map is not that fancy but its enough for newcomers to understand

Los santos - GS-9
Red county - GS-11 North montogomery, Gs-12 Dillimore
Flint county - GS-10 Close to flint intersection
Whetstone - GS-8, south-east of angel pine, GS-7 south-west Angel pine
San fierro - GS-5 Calton heights
Tierra robada - GS-4, east of Las Quebodros PD, GS-13 North-west of Las barranas or west of cluckin bell
Bone county - GS-3, West of LV whitewood estate area
Las venturas - GS-15, West of LVPD, GS-1 South LV camel toe, near ammunation
Some gas stations didnt have any number at /area, Example the gas station at temple or gas station at SF CJ garage
Any gas station which is missing at the map ( must have number ), please inform me