The OP specifically asked for alternatives "rather than make a starter every time I'm about to brew". Plus there are lots of other advantages to canning starter wort.
1. No need to do the mix-boil-chill hassle every time you need a starter.
2. No need to keep DME around. When I used to handle that stuff, I could never avoid making a giant mess (plus it's expensive).
3. If you're culturing yeast, you've got "sterile" starter wort for the intial steps. Boiling only gets you "sanitized" starter wort.
4. Gives you more to do with your pressure canner
and it's really not much hassle to mash up a small batch to get your wort collection started. You don't even have to boil it - the pressure canner takes care of that. Once you've got some jars canned up, then it's an easy thing to just bump your batch size up slightly each time you brew and can the extra for more starters.