www.crloo.com has lauscha 104 tubing. If I were going to buy only one size of tubing I would suggest 18mm. The lauscha tubing is hand pulled and your diameters will vary. Order a size as close to 18mm as you can get.
If you are feeling flush and want to order more sizes. I typically use 10mm with matching 10mm ornament caps for the mini ornaments. 15 or 18mm for the medium and long ornaments, and 25mm for the round ornaments. You can make nice round ornaments( but a little smaller) with the 18mm.
Arrow springs does have some 104 clear tubing called chandelier glass. But it is a larger diameter than the 25mm and it is also heavy wall. It might be nice for the round ornaments and make really big ones. I have some ordered but have not tried it myself yet.
hope this helps,
Candice