You could try http://www.wdmodels.com/ for accessories such as fuel cans, boxes, barbed wire etc. For machine guns, there are any number of plastic figure sets containing vickers or spandau, of varying quality. For high quality (and cost) there are the mini-world guns, or you could try aircraft accessories in etched brass, for example Tom's Modelworks do a few sets which are very detailed but equally fiddly.
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