http://aebike.com/page.cfm?PageID=30&action=list&Category=556&brand=367&modelid=3336&type=Thttp://aebike.com/page.cfm?PageID=30&action=details&sku=FW8428http://aebike.com/page.cfm?PageID=30&action=details&sku=FW8421The above links to AE Bike shows Shimano HG50 cassettes in 9 speed in 14-25 and 11-34. HG50 is also called Tiagra. HG70 has the same configurations and is called 105. Nashbar occassionally has various configurations of HG70 cassettes.
The 14-25 has 14,15,16,17,18,19,21,23,25 cogs. The 11-34 has 11,13,15,17,20,23,26,30,34 cogs. I would take the 14,15,16,17 from the 14-25 and mate it up with the 20,23,26,30,34 cogs from the 11-34. Nice 14-34 9 speed cassette.
HG50 is champagne colored. Brown. HG70 is chromed, nickel plated, shiny silver, whatever. All of these are loose steel cogs with individual spacers between all cogs except maybe the first one where the spacer is part of the cog itself. In real life use you will never know the difference whether you are using low or high end Shimano cogs. Same steel in all of them. The higher end cogs have aluminum carriers to reduce weight. Big deal on a touring bike. The ramps on the sides of cogs to speed shifting will be the same across all of the products so they will all shift the same.
On my road bikes I use the cheapest Veloce or Mirage cogs I can find. All loose steel cogs and plastic spacers. Mirage is galvanized steel instead of chromed on the Veloce. I don't care, the galvanized Mirage were cheaper. And shift just as well. I have Chorus, Centaur, Record equipped bikes. I believe in cheap chains and cheap cassettes. The wear items on bikes. Nicer shifters and such.
I think you can mix and match a limited number of 8 speed cogs in a 9 speed cassette. But they are different thickness so you only have room for about 1 or maybe 2 8 speed cogs in a 9 speed cassette. Better to just buy a 9 speed cassette and not worry about such things. They are only $30 full price for the HG50 cassettes. QBP sells them so your local bike shop can order them.