If you have a true 7 speed cassette (if it is an 8 speed with a spacer, you can just remove the spacer and fit an 8,9 or 10 speed cassette on it), you can take a 9 speed cassette, disassemble it, taking the last cog off (the lowest speed cog), and putting it on the 7 speed cassette hub. You would need a 9 speed shifter, which you can still buy from Nashbar and other places, and you'd need a 9 speed cassette which you can disassemble, which means 105 or lower quality. But it works, and you get 8 speeds without having to change wheels. Further, it sets you up for 9 speed if and when you do finally change your hub.

I had this setup for several years on my commuting bike; it worked perfectly, and when I replaced the hub, all I had to do is adjust the limit screw on the derailleur to make it a 9 speed.