I suspect the belt you want is a V belt type Z32. This has an internal length of 815mm. I got one over the internet from www.bearingboys.co.uk for £2-26 with £3-50 postage. VAT is on top of both prices.
They supply a vast array of v belts of different Z numbers (and hence lengths), so should be able to supply for most mowers.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.