I just received a M&P 22 magazine, I paid extra to get the California kit . The so called kit was they took the the spring out of the mag so they could legaly sell it to me, they did not include the spacer to limit it to 10 rds. Can you buy this spacer ?
I made one out of a 223 casing but dont know if thats legal

In order for it to be legal it has to be a permanent change.
Example - A part that is riveted in place or a part that if removed won't allow the mag to function.
The M&P22 Mag doesn't have a spacer it has a pin inside that limits the follower movement.

The law...
CA-Legal Magazine Repair / Rebuild Kits are to be used to 1) repair or rebuild your existing legally owned high capacity magazines or 2) permanently block to 10 rounds or 3) store in a disassembled condition for use outside of California. They are not intended for reassembly into new high capacity magazines, which would be a felony under California law.