There are a few ways the schematics for bottle cap mines can be acquired. You can buy it from Knick Knack. You can find a schematic in Jocko's Pop & Gas Stop. You can get it as a reward from Herbert Dashwood if you tell him the fate of Argyle. You can get it as a quest reward from Moira Brown ('The Wasteland Survival Guide' quest)if you complete the additional objective.