Option 1
You can use the for loop:
for file in ~/.alias_*; do
source "$file"
done
Option 2
Another option is to build an array of all the files you want to use, and then iterate over the array using a for loop.
typeset -a aliases
aliases+="~/.alias_foo"
aliases+="~/.aliases_bar"
for file in $aliases[@]; do
if [[ -a "$file" ]]; then
source "$file"
fi
done
zshrc.