You can pick up chair umbrellas at Wal-Mart for around $10 each and they work OK. The post is a bit long, though, so you need a good quality stand to hold it in place, especially if the wind is picking up.
I am generally not a huge fan of umbrellas on the mics due to the amount of wind resistance and noise from the drops they pick up. If you have a poor quality or a very light stand, they also function as sails. Use condoms fitting over the entire mic for rain protection. If it is raining so hard to where you need to use an umbrella, it is better half the time to call it a day and just put everything away rather than risk damage to the equipment.