I'm not positive, but I'm fairly sure you can use a 9sp cog on an 8 speed cassette, so long as you've individual cogs and spacers. I found a few 9 speed 28T cogs floating round for $20.

Another option is a smaller chainring up front, which would have the advantage of retaining closer gear spacing in the rear, if that matters. It's also probably a less expensive route, so long as you can fit the smaller chainring on your current cranks.