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.http://www.instructables.com/id/Czech-dumplings/
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.