Try http://www.svcyclingcenter.com/bikeconcierge/bikeconcierge.php I think this fits what you're looking for, but be prepared to pay serious $$. Otherwise you can check out the local shops that demo bikes. Calmar in Santa Clara will demo Spesh and Santa Cruz for $50/day which I think you can use as store credit. Passion Trail Bikes is pretty far, but they demo a variety of high end bikes. There are probably others, but those are good starting points.
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