How can I buy a customize item?

I wanna buy an customized item

You can’t.
You can however buy gold, spend it on the 5.43mil gold random crystal in the shop (looks like a jasper), and rinse a repeat as many times as is necessary to get enough crystals to craft your own. Or just play more and eventually earn them for free. “Converting” crystal and Eternal legends for ~500k gold also gives random CS from jasper to obsidian. Good luck!