Laws vary by country and in the US, by state. Most, if not all, states require one mirror.
For US laws by state, check the AMA site.
France - two mirrors required
UK - none required.
The mirrors on the F model are fairing mounted, though the mounting holes for the E-style mirrors still exist. Some owners reported satisfaction with the ES-5 model available at Ride Gear.
Mirrors from the SV650S, SV1000S, and 2004+ GSX-R will work, but have shorter stalks.
The stock mirrors have a 10mm mounting hole on each side. Be careful of using mirrors from Yamaha and Ducati, which may use left hand threads. Here is the stock mirror:
These are a popular choice, as they can easily by purchased for $30/pair on Ebay, or $70/pair from a Suzuki dealership. Also check SV Riders and Southern CA SV Riders in the classifieds section, where you can expect to find them for $40/pair, including shipping.
The 1st generation SV650 had round mirrors (also called Mickey Mouse, mouse ears, and lollipops), as shown here:
The 2nd generation are rectangular, like the GS, but have multiple pieces welded, rather than a single bent segment. Many owners prefer this look. SV650 mirrors are convex (giving you bigger field of view) and they are also fog resistant. (Try to breath on such mirror, it will not fog. Stock GS500 mirrors do.)
Very well liked, but also very expensive and rather flimsy. Made for racing. These have adapters so they work on both faired and unfaired bikes. Very good looking but can snap easily. Available from WHA for $65 each.
Shorter than stock, they lower the overall profile of the bike. Cheapest supplier is Ride Gear, for $24/each. Recently, Ride Gear's inventory of these mirrors is sold out/discontinued. I found a pair at Bob's Cycle Supply for $21.99 per mirror.
Available from Chaparral Racing. $12 each.
While cheap (as low as $6.95 each), they were advised against by many users. Complaints include excessive vibration, drift in the position, and the mirror glass coming loose inside the frame. They are available in long or short stem.
There are several different mirrors, from $10 each to $130 each, depending on your budget and feature list. Search through motorcycle accessory catalogs or online at sites such as WH Accessories.
The GS handlebars from 1989-2000 had a threaded insert in the handlebar ends for the bar end screw. As a result, bar-end mirrors that require a hollow handlebar will not work. You can either switch your handlebars for stock bars from a 2001+ model, purchase aftermarket handlebars that are hollow, or modify the bar end mirror.
Several owners purchased bicycle bar-end mirrors, such as the Third Eye, which are available for approximately $12 each. Several report excessive vibration, though others claim that vibration can be reduced and even eliminated by proper installation. "Fairly tight without having the plastic bracket touch the end of the handlbar (use the rubber insert as a sort of buffer)", says GSTwins member wasabi_peas.
Others had problems with cracking and snapping of the bracket.
GSTwins member 500rider recommends going with a glass mirror instead of the plastic aluminized film, as the latter scratches easily and is not very clear.
GSTwins member Shadowhawk recommends the Catseye mountain bike mirror, and says there have been no vibration problems.
These are available at most bicycle or outdoor shops.
The cheapest bar-end mirror available. You can mount them above or below the bars. Some prefer below to lower the profile of the bike, but at full lock they will hit the frame.
Cheapest supplier: Bike Bandit for $28.76
For more pictures, see this thread
GSTwins member chimivee used an aluminum L bracket and a 1" convex blindspot mirror to create bar-end mirrors that are small, unobtrusive, and legal.
These are highly regarded by forum members, but pricey. They are designed to fit around a 7/8" bar, so you have a few options:
Available in 2" ($44 each), 3" ($68 each), and 3" folding ($83 each), which is useful for lanesplitting. The adapter is $18 (each).
Thought to be made by Motrax. GSTwins member Chilly Willy speaks highly of them.