Maintaining a large stock (a war reserve) of ammo is never popular. Thats because the stuff has a short shelf life. Missiles and artillery shells degrade over time, even with maintenance. The problem is that the chemicals that are used for the propellants (in missiles, artillery or small arms ammo) and explosives (missile warheads and artillery shells) are unstable and degrade over time. Most missiles are built to last, with proper care and storage, for ten years. Artillery ammo, depending on the component (fuze, propellant, explosives), lasts 5-20 years. As the stuff gets older, even if well cared for and not past its expire date, it becomes less reliable. When it reaches the expire date, you usually fire it off in practice. This provides good, but expensive, training. Keep it beyond it's expire date, and some of the stuff becomes downright dangerous for the users.
The Taiwanese legislature would have no problem with paying for emergency deliveries of missiles, and they know that the fighting in Iraq has not depleted American inventories of missiles Taiwan would need (air-to-air, SAMs and anti-ship). Perhaps arrangements have already been quietly made to make those air freight deliveries. But these may not be quick enough. Taiwan only has about 900 Sidewinder air-to-air missiles on hand, for over 300 first line jet fighters. China can mass over a thousand fighters on their side of the Taiwan Straights for their initial attack. Taiwanese pilots are better trained, and perhaps expect to take down many of the less modern Chinese fighters with cannon, instead of missiles. That can be dangerous for the attacker, getting in close like that. But with their small supply of missiles (the Taiwanese also have nearly a thousand air-to-air missiles of various other types), cannon will be the only option after a few days. But China has several thousand fighters it can send in to replace losses. In a war of attrition, Taiwan could lose control of the air.
Taiwan also has low stocks of artillery ammo, although they justify that by putting lots of effort, and money, into building up their air and naval forces. The Chinese cant walk on water, they must control the air over the Taiwan Straights in order to get troops ashore on Taiwan. But theres always the chance that the Chinese marines will hit the Taiwanese coast before the American missile resupply flights arrive. If you don't have a lot of artillery shells, you can lose the ground war as well.
Taiwans defenses may fail not because of a lack of weapons, but because of a lack of ammunition. The Taiwanese legislature has been stalled over proposals to buy more (very expensive) new weapons (eight diesel-electric submarines, 12 P-3C Orion maritime patrol aircraft, and a dozen Patriot air and missile defense systems). But now another procurement issue has flared up in Taiwan, the lack of ammunition. Taiwanese media is awash in stories about ammunition shortages. The United States has long warned Taiwan that it needed more missiles, in particular, if the island were to hold out for the 5-10 days it would take for American naval and air forces to arrive. No one ever came right out and said it, but the Taiwanese seemed to assume that the United States would immediately fly in thousands of needed air-to-air, SAM (Surface to Air) and anti-ship missiles once China attacked.