Where does “小泉” come from?

小泉 (Japanese) comes from Japanese 小, from Old Japanese ささ.

Ancestry of “小泉”, step by step

Step	Language	Word	Meaning
1	Japanese	小	smallness; small size, small; Short for 小の月;...
2	Old Japanese	ささ	—

小泉 — Etymology, Origin & Word History | Etymology Explorer