If your inverter is DC to AC and DC input voltage is around 12V or 24V, you need not to use a 500V transistor, a 150V transistor will do the job efficently, why pay more cost?
Perhaps you should have a look at one of your local electronic retail pages. Here in Australia we have Farnell and RS Components - each of which have a parametric search to narrow down your component selection criteria until you have a list of all the compatible parts (then just order the cheapest). I'd assume you should have something like this near you?