Yes, you can take oleander together with probotics and Vitamin C. In fact, the only supplement that is not compatible with oleander is Protocel.
I would suggest googling "organic nurseries" in your area. Regardless, always ask to make sure that they do not use pesticides. If you live in the Dallas area, Tony recommends the one below.
And thank you so much for your kind compliment!
I think that Rohdes Nursery (the ones at beorganic.com) will ship anywhere in the US, though certainly it makes sense to find an organic nursery close to where you live.