I can purchase Antminer D3+ at good price, but they sell it without PSU.
I have Antminer S7 PSU to supply the Antminer D3+?
In theory it should work. The Antminer D3+ uses less than 500 watts whereas a S7 power supply provides over 1000 watts.
You should make sure that the power supply is compatible in other respects. You need to make sure the S7 power supply and the D3+ that you are considering are made for the same voltages and frequency (which vary across the world)
Bitmains only manufactures one PSU. See here... https://shop.bitmain.com/productDetail.htm?pid=00020170920224132179WEIOvhXx06D6
It states at the top of this page it is good for an Antminer S9, D3+ or L3+. So, you will be fine using the one you have on the X11 unit.