解：?$(1)$?設甲種樹苗買了?$x$?棵，乙種樹苗買了?$y$?棵.
?$\begin{cases}{ x+y=500}\\{ 50x+80y=28000}\end{cases}$?解得?$\begin{cases}{ x=400}\\{ y=100}\end{cases}$?
∴甲種樹苗買了?$400$?棵，乙種樹苗買了?$100$?棵.
?$(2)$?設甲種樹苗買了?$a$?棵，則乙種樹苗買了?$(500-a)$?棵．
?$50a+80(500-a)≤34000$?
解得?$a≥200$?
∴甲種樹苗至少選購?$200$?棵.
