Would love some guidance on this. Trying to get the right spoke size to rebuild wheels on my 1975 DT175B
From my measurements, I think my spokes are about as follows. See pictures.

Front 168 / 176   (19" rim)
Rear 169 / 184   (18" rim)

These measurements were taken from my spokes from the original build. They all had about 3mm of thread exposed so I'm wondering if I should add that on to the measurement if I get custom ordered ones or if that just has to do with how far up the threads are cut. There are a bunch vendors on eBay and the Internet that sell spoke sets for this model and year, but sizes don't match up - not even close - especially the front. They just say, "yea yea don't worry, they will fit." I assume these hard measurements can't lie but I'm just curious if anyone has any suggestions to order the correct spokes and from where?

Buchanan's Spokes here in the USA also I’ve had good luck buying from vintage_001 on eBay bought several sets from them and they fit fine

Buchanan's.

Yeah but what size are they?

I can’t answer your question as to what size are needed but would also like to recommend Buchanan’s. They just needed motorcycle info and always sent correct spokes. One was to lace a 73 CT 175 front hub on to a rim off a ‘77 250 rim and they made up correct spokes just using the model tag information.
Don’t know how they can do that but was impressed. 
 Not sure how their prices compare to others but for me it is hard enough lacing spokes happy having them figure out what is needed to fit. 
 

If you are rebuilding the wheels using the original rims, and all was ok, why not just try and find a wheel builder, who can roll spokes, and order spoke sets exactly the same dimensions as those you removed?

Here in the UK the big wheel building places can supply spokes, but they take several weeks and charge high prices.

Thing with spokes are not just the length, but the correct angles.

Call Buchannan's. (buchananspokes.com) They knew which type/sizes I needed for my bike before I told them my bike's OEM spoke measurements.  Get pictures of inside and outside spokes so they can clearly see what you need.  I remember the guy asked for an email while I was on phone w/him.

Good idea to take a picture of a few spokes with spoke laid next to measurement so they can see what they are.
 

Are you asking Size as in thread diameter say 4mm & thread pitch say 0.8 [just using examples but not correct ones].