Does the oven door need to be toughened glass?
You can't use just any sheet of thick glass for your oven door. Glass for your oven has to be tempered to resist high temperatures. Untempered glass will likely crack pretty quickly with regular use. Moreover, an oven door usually made of glass which breaks into "rough pebbles" rather than sharp shards if it breaks, to decrease the risk of injuries if the glass does crack. If you replace it with a regular thick glass pane you run the risk of (worst case scenario) very sharp, very hot shards of glass exploding all over your kitchen one day.

Like @M.K. stated in the comments, your best bet is to contact the company and ask about replacements, or a reputable company that repairs appliances. They can easily secure an appropriate piece of tempered glass to replace the broken one.