Does the TMG replace the
Flourish, or should I use both?

If you are the one who bought the KNO3, K2SO4 and MgSO4 from me, then
I think you did not get the extra Trace mix.

So use Flourish *or* TMG with KNO3, K2SO4 and MgSO4.

On the subject of CO2, I plan to use DIY. Where is a good mail-order
store
to pick up CO2 airline tubing, or can I use regular tubing for a
while?

Initially you can use regular tubing. I think BigAlsOnline.com dosnt
have small length of Silicon tube. You can try www.thatpetplace.com.

also need yeast, brewers yeast or Phyl at one point suggested champagne
yeast. I contacted a mail-order home-brewer supplier and he said "sure",

Sorry to suggest local business, but its really cheap to get it
locally and your mailorder yeast may be expensive.

Thanks
Dominic