I don't know what kind of tees and elbows you got, but we run into this
sometimes with plumbing and A/C fittings.What plumbers call 1/2" copper,
we (A/C) call 5/8, same for 3/4 vs 7/8, and 1" vs 1 1/8. 1/2" ref copper
is rarely used in plumbing and I don't thnk 3/4 ref is ever used. I do
not think plumbers use many 3/8 fittings other that compression or
flare. I have never noticed them in the plumbing dept at HD or Lowes,
though I must admit I never really looked hard for them. If you cant
find what you need there or at a plumbing supply, then try an A/C supply
house. Take a sample of an inch or so of your tubing to make sure you
are getting what you need. HTH Larry