There are none on the Chambers website so your choices are these:
Order one for the Baikal 512 air-rifle (if it has a breech seal!) which is what the IJ-38 developed into. It might not fit though! Ask York Guns, who sell these modest but sturdy rifles.
http://www.yorkguns.com/baikal-mp512...ton-rifle-wood
You can make one from leather using a couple of punches or some other improvised tool.
You can find an 'O' ring which fits snugly, and pack the depth with slices of another 'O' ring or card or whatever you like. This is the Ian Jones 'BSF breech-seal' technique and it works well.
You can use a breech seal from another rifle with similar dimensions.
I have the same problem, so I will let you know!