One of these two Pokemon has to be Bulbasaur/Ivysaur/Venusaur otherwise it is not possible to breed a Bulbasaur.
If it is a female you have to breed it with a male Pokemon from the Grass or Monster egg group (click links to find out exactly which ones).
If it is a male tho, the only two Pokemon it can breed with to get a Bulbasaur are:
>1.A female Bulbasaur/Ivysaur/Venusaur
2.Ditto
Source - Experience
Hope I helped!
