Any recommendations on who/where to approach for customizing certain design for baby romper? And also where can I print if I managed to find a ready template?

There is quite a lot of printing companies that you can approach. I used to print one set of family tee a year for my husband as wedding anniversary gift. This is what I did for last year. The printing shop that I approach will normally use Mothercare baby romper for the printing. Some shops even do free delivery to your house after printing. You can source and enquire on the material that they used before you decided to print. :-) Alternatively, you can try Toddley Thought. They have some nice recommendations/feedback online but I personally never used them before as I prefer to design my own template. http://toddleythoughts.com/about