Alternatively, if money is in short supply, you can (usually) remove the stock-fan from the heatsink and oil it.
1} - Unplug the PC from the mains power, then open the case.
2} - If you look carefully at the fan, you should see 4 screws, 1 in each corner. Follow the fan-wires to the mobo and unplug it. Remove the screws without dropping them onto the mobo and lift off, WITHOUT removing the heatsink from the CPU.
3} - While the fan is off, clean out any dust-bunnies from between the heatsink fins, and clean the fan-blades with a small stiff(ish) brush.
4} - On 1 side of the fan is a label in the centre, very carefully remove this. Some fans just have an open hole underneath the label, some have a plastic/rubber disc covering the hole - Carefully prise it out with a pin, fine-bladed knife or something.
5} - Apply 1 small drip of light oil (sewing-machine oil is great) to the fan-bearings in the centre of the hole.
6} - Re-assemble - Same as removal but in reverse - Oh, and without replacing the dust-bunnies
If the label tore during removal, cover the hole with a small piece of vinyl tape or cellotape, just big enough to cover the hole but not extending beyond the blades-hub.
Your fan should be quieter and you'll have saved some money. If it's still noisy, then I'm afraid it'll be the options listed above, it's worth a try though - My 1st home-built PC's fan lasted approx another 12 months, using this cure.