As an event planner, I use Classic Party Rentals in Burlingame. They have standard Riedel glasses, and the "O" glasses if you are doing upscale wine service. Any event rental company in SF, like Abbey, will have all purpose Libby wine glasses, but they have a slight edge on the lip of the glass.
I third (fourth?) the recommendation to buy some. It's as cheap as renting them, and even if you aren't going to use them again you can sell them or give them away.
In that vein, if you have time to browse Craigslist, people often sell their wine glasses after throwing a big party. I just noticed a guy last week selling 120 wine glasses in Berkeley.
Also check out Action Rentals on Folsom. I've rented party tables and chairs from them - maybe they have glasses.
But I second dolcevita sf's recommendation for the Ikea glasses. I love those Ikea wine glasses - they are $5 for a six pack, made in the USA and practically non-destructible. They go in the dishwasher and store in their stackable six pack boxes when not in use. I have 4 or 5 six packs of the champagne flutes and also of the red wine glasses, which I use as multipurpose wine glasses. Great for big parties.
I know several people who've decided to buy glassware rather than renting it, because it can be cheaper. Ikea has red or white wine glasses at $4.99/6 http://www.ikea.com/us/en/catalog/products/30015123
Classic Party Rentals is a reputable company that does tons of weddings and events in the Bay Area. (They are located in Burlingame.) Their price list indicates that classic stemware rents at .55/each