Why do you need an alternative to a conventional compressor? If the answer is, "so you don't have to spend a bunch a $$ on a tool you will only use once," consider the following options.
Option 1: Go to Harbor Freight, buy a valve spring compressor for $6. It's not a very good one, but you won't be using it every day.
Option 2: Go to Autozone or equivalent retail auto parts store and see if they have a valve spring compressor in their lend lease program. It's free!