You can try something like, get the file list which are matching some extension, like .txt or .c.
svn list -R http://svn/url/till/the/path/you/need/ | grep ".extension"
Then for every line in the output of the above command use svn export
to get the files in your local machine.
Edit:
repository=http://svn/url/till/the/path/need/
$target_directory=some path in machine
for line in svn list -R http://svn/url/till/the/path/need/ | grep ".extension"
do
filename=`echo "$line" |sed "s|$repository||g"`
if [ ! -d $target_directory$filename ]; then
directory=`dirname $filename`
mkdir -p $target_directory$directory
svn export --force -r HEAD $repository$line $target_directory$filename --username abc --password password123
fi
done