Hi 
I first had a 79 DT250 in 1979 as a 17year old , so now at 60 got my hands on a do upper  but parts are hard to fined for the UK. spec. 
. For an example got a side stand spring on line for a 1979DT250mx but its to long ..
cheers

Do you have a twin shock model? If so, it may be quite similar to a 1976 US model. Having the model code number (possibly the first three digits of the VIN stamped into engine and frame) is often very helpful.

We have a great bunch of members in the UK so perhaps some of them can help out.

Greetings from Tennessee. 

 from near Salisbury, Wiltshire.

First thing I'd do, if you haven't already, is find genuine UK/Euro parts manual and service manual for your model - hopefully available on fleabay, although you might have to buy from France or Germany.

You can then put those numbers into the CMSNL search box to see if they have new items available. Also search fleabay under the part number(s) although, as you have already found out, you need to be careful that the parts are correctly advertised - researching photos of the parts on CMSNL and the internet in general can help with this. 

 from Berkshire.
Fowlers in Bristol are good for lots of OEM parts although you might need the part number as their search engine isn’t great.
Yambits carry lots of non OEM parts.
I use CMSNL a lot to check part numbers and I have ordered from them.
This 1980 manual might be helpful yamaha-enduros.com/index.php/service-man...ners-manual/download

Obviously(?) Yambits and CMSNL are useful resources. For the latter I get parts from the US spec parts lists…
but check part numbers first. Ebay can obviously be fruitful too… I have saved searches for DT NOS and get the occasional useful / rare part. But have been looking for a tail light damper for weeks without success… so what do I know?!