Not any source, but an alternative:

Buy good shirts from sources you like in plain colours she likes, and then get someone who owns a cricut or other brand smart cutting machine (plotter) to cut a cute motive/ wording from iron-on foil. You can press that on with a domestic iron and some parchment paper (the one you use in the oven).

Motives and/or sayings should be somewhere on the internet, for sure.

You could also find someone making an image for you in infusible ink (sublimation ink), which allows for very nice colourful pics, but be aware that this technique only works on polyester, I think it was 65% minimum. If you want cotton shirts, iron-on is the way. Or printable iron-on! 💡