I'm having a mare getting some 'straight' M8 threaded rod (to use as a leadscrew - we're not all loaded!)

I'm now on to my 4th length (& this is starting to add up - so much so, that it'd probably have been as cheap to by a proper leadscrew!). So far, I've had these scenarios...

warped rod to start with (ie from the seller),
a (likely) straight rod but Royal mail sorted that out!
me thinking a rod looked straight at a local hardware store, but actually itwasn't when I put it & put it in my machine....

So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die (I need one about 44cm long...I think it's this relatively the length that is proving to be the problem)