Not particularly cheap, but I have purchased several great quality NOS vintage Eulit Perlon straps from timesofplenty via eBay in the US. No import charges so far, a very 'customs friendly' service. Last time he threw in a nice freeby pull-through nylon strap as well. I really like the straps, very comfortable and some really interesting colour options.
http://www.ebay.co.uk/usr/timesofplenty