The best flour to use when making Spaetzle is a rather coarse one. It will make the finished noodles a little more chewy.
So you can go out and start searching for a coarse ground pasta flour or use a mixture of all purpose and semolina 1:4.
I described the type of flour used already in an older Ible.
If you don't have semolina either you can as well take usual all purpose flour alone. But the noodles will turn out a little softer.