The easiest way to get Subversion is to get a prebuilt package from your operating system supplier.
If you can't do that, you can get the source code from the Subversion website and build it.
Note that Subversion is much more than a way to download code such as Netpbm, so you may read warnings about how complex Subversion is. These warnings generally don't apply to the simple use you want to make of it. For downloading Netpbm, building and installing Subversion can be a lot simpler than the general case.
For one thing, there is only one program you need, called svn.
Subversion normally uses a pile of shared libraries, but since only that one program would use them, you can simplify matters considerably by using statically linked libraries instead. To do that, use the --disable-shared option to configure.
There are various prerequisites the Subversion build wants, including a Berkeley database manager. You can ignore warnings from configure when it can't find these, because they aren't needed for Netpbm downloads.
If you don't need a language other than English, you can simplify things a little more with the --disable-nls option.
Finally, you can speed things up a little and minimize your exposure to build problems by building just the parts you need:
./configure --disable-shared --disable-nls make -C apr-util make -C apr make -C neon make subversion/client/cmdline/svn cp subversion/client/cmdline/svn /bin/
Even simplified, the Subversion client program is enormous, and it takes about as much CPU time to build as all of Netpbm.