There are several sources... I personally prefer Dharma
I highly recommend going to the Dharma site and reading all their help and description areas
Momme MM as in 5MM or 8MM is a weight and weave standard...8MM is a tighter and heavier weave than 5MM
That said, I have yards of both the slightly lighter (per yard) 5 MM....but seems to me, to need a tad MORE dope to fill than the heavier 8MM so final finish weight is about the same within a few grams...this is just my opinion based on how I determine final fill and glossy
Note: the 5MM stuff can be bought cheap and white...almost all the 8MM stuff is colored.... Dharma has excellent series of how to's on all their different dye methods
Since for me, there is no reason to stick with 5MM white and then the expense of time and cost for dyes...My opinion is to not worry about 8MM being heavier if they have a color that fits your final color scheme
As much as I would love to support the Cottage Guys their silk is way too expensive....Thai or Dharma have plenty of stock in all sorts of colors well under $5 per yard
ONE hint
Thai or Dharma silk shrinks 1 to 2% in width and 6 to 7% in length...DO NOT BUY a 1 yard/36" by 1 yard/36" SQUARE piece...near impossible to determine length vs width....suck it up and just buy two yards...easy, still cheap, done, and no problem NOW knowing how to cut it for the wing panel....
Hint #2...today's silks do NOT shrink as much as the 1950~1970s stuff and a lot of it NOW has sizing chems added...
SO pay attention to orientation
Pre wash/rinse in clear hot water...no soap
Let dry if later project... as drying--- stretch it in both directions
DO NOT apply the silk sloppy wet...just barley damp and stretch as you go doping down the perimeter
Use Butryate Tautening for the initial first two or three fill layers then switch to NON Tautening