If brokers don't let you buy a stock, that's because they know something you don't like the price is going to take off. So they want the shares only for themselves. They are not working for you, contrary to what you may think.
Once the price is way up and they have accumulated enough shares, they'll let you back in the game.
Have you noticed that when you buy a stock its price tends to go down and when you sell a stock, its price tends to go up (at least short term). That's because market makers hate to lose money and they are the ones who set the price. When they sell you a stock, they want to be able to buy it back cheaper and when they buy a stock from you, they want to be able to sell it with a profit. Indeed when you buy or sell a stock you buy or sell to a broker or market maker and not from another individual investor (that's why executions are so fast for market orders).
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