Look for some new made Fiocchi 5.75 Velo Dog ammo and or brass, it is perfect fit for the Maynard XL, Fiocchi new and fired brass needs resizing, bullet size is about .229" diameter.
BACO Usually has brass, and of course RMC will make them to fit your chamber, if your bore is .224" you can partial neck size & seat with Hornet dies.
If .228" bore CH4D is your best bet.
Brass availability:
(You need to Login or Register to view media files and links) If 22 XL rifle is original it more than likely has a .228 bore, think RCBS makes a limited run mould, NOE has some .228" moulds
(You need to Login or Register to view media files and links) Loading data for the 22XL has to fit your rifle, chamber and bore, with such a small capacity case a couple tenths of a grain over can be too much, I'd go with 4227 or maybe 2400; Unique can go over very fast in these small capacity cases.
Of course if a Stevens 44 / 44 1/2 you can breech seat with a plugged case or use pusher type seater and only need one or a few cases, bullets will probably require a mould or maybe someone on the site can get you a few to test with.
Good luck with your rifle C.M.M., if I ever find the photo I have of the small 22 cases lined up including the Maynard 22XL I'll post it.
Mike