Even if bikes have low miles there still can be issues with dropouts etc.. my suggestion is that if the bike will shift in one direction and have troubles going the other way then there are a couple of issues. First, there is always the cable tension issue, but the logical part of me says it can also be the spring in the rear derailleur is worn out. I know that we think with low mileage bikes that this shouldn't happen, but as we all know dirt/grit do a number on bikes. The way to check this is to pedal the bike in the stand and manually push the derailleur to the opposite end of the cassette(do not use the shifters to do this). If the chain falls back to its orginal spot quickly, then that obviously isn't the issue... but if it tends to hang up and be very slow then replace the derailleur along with a new cable/housing.

Now as far as cassettes, Shimano and Sram should work fine... although I will admit that I prefer SRAM. It's very difficult to buy parts this time of year simply because all of our suppliers are waiting on orders that might take a bit... remember in bike world we are already in the 2013 calendar year. For future, I would suggest that you always have a spare chain, cassette and brake pads... that way if you're travelling for a race and the local shop doesn't have what you need then you'll be all set. I know it can be expensive, but it can save all sorts of stress in situations like this.

If your cassette doesn't arrive, why not ask whether the shop has a cassette on a new bike that you can buy and replace it with yours when it comes in. Some times this time of year, it isn't that the shipping is a problem it's that the amount of our mimimum orders are sometimes too large for this time of year... I've also had some issues like that in the past couple of weeks. As a shop owner, do you spend $150- $200 when you only need one or two items?

I do hope that your cassette arrives in time, and good luck in your race!! Let us kow how everything turns out for you.