Generally, there is never a plethora of rentals in Orinda, regardless of the time of year--it just does not have a large base of rental stock. However, as long as one avoids the "dead season"--November through January--it is usually possible to find something. I have found the best resource currently to be Craiglist.org for virtually all types of rentals. And now it appears that Trulia is working on increasing the number of rental listings too. A small percentage of rentals is handled through the multiple listing service, and occasionally a very high price rental will be marketed through specific realtors.
Hope this helps!
Linda Friedman, Broker