I found an 11-34 7-speed Shimano HG cassette about a year ago with a lot of searching on the internets. Don't know if there's any still out there. It goes 11,13,15,18,22,26,34

Looked thru my e-mail history. Got it at www.niagaracycle.com
1 x Shimano CS-HG50 Mega Range Cassette 11-34T 7S Silver (90231) = $9.79


8sp HG cogs are about the same thickness as 7sp (as long as you dont have 7sp IG cassette) so you could also buy an 8sp cassette and take it apart and use a few cogs from it with your current 7sp cassette.