Might be Allamanda cathartica

Spectaluar tree. There is also a double flower one, peony like, which is rarely mentioned. Please take note.

Penstemon cobaea, I believe. Some may be more purple, while others are whitish, such as the ones in your photograph.

Kerria, forsythia, mahonia, hypericum will all produce yellow flowers but none of them are very large. They tend to make up for lack of size with abundance :-) Try Googling with the botanical name and seeds - that should provide a number of sources (it did for the kerria and hypericum). But I too would start out with a small plant........growing just about any woody plant from seed is a long, multi-year process to get to a flowering stage.