Don't know about the best source, but there is very little variation in T5 lamp specs, certainly between all the branded lamps. Your main choice in 4' tubes is between the standard output 28W at about 2900 lumens, and the high output 54W at 5,000 lumens. You will of course be wanting the 54W tubes if you need T5s. If you don't have high light plants then consider either the standard output 28W T5s which are more efficient, or use T8s which are much easier to find and cheaper. Lumen maintenance will be about 95% on all T5HO tubes, lifetime will be 20,000 hours or so, CRI will be 85-ish. There is a very small difference in specs between some unbranded T5s and the best branded ones but not a lot. Using a good programmed start ballast will also make a small difference.

Shop around, the best deal will probably be the one with the cheapest shipping.

The brand name Phillips, Sylvania, Osram or GE are the most common ones and they are quiet equal in quality and lifespan. What you would want is a CRI as high as possible as Shrub n bulbs mentioned 85 and up and the higher the lumens the greater the coverage (2000Lumens/sqft min. or 5000Lumens/sqft optimal. In your case as the plants are very young and will be out in the sun after reaching 12inches I would recommend to go with the min. reqÂd to save money) as to seedling you will need very little light however for vegetative growth you would do best sticking to a colour temperature of 3100K.