My aspirations in this area, for my toddler girls' line, are high. One aspect of high-quality knit fabrics is that striped knits are yarn-dyed rather than printed.
Having a yarn-dyed fabric custom made for a small boutique line is virtually impossible, because of impossibly high minimums. But there are some wholesale sources that can provide good yarn-dyed stripes. I m finding them gradually online, and will soon be incorporating them into my striped designs.
Some of my favorites are Denver Fabrics and Sand Textile. If you find any others, please let me know!