You may as well just keep on saving, & upgrade the alloys when a couple of your current tyres wear out etc.
If you can't wait, then I suggest you either buy a nice set on a credit card, & transfer the balance to a 0% card (there's loads of them which last for 9 months, too), or get a low APR 'high street' loan, as wheel places often sting you on their own finance with high APR's.