Aye Joolz,
It would appear that instead of using lock tabs for the rear sprocket nuts, Honda are now supplying NUT, U, FLANGE. 10MM, #90304GBL003, Replaces #90304GE8003
These are available from David Silver and I have posted the link to their parts list below:
Honda_XBR500_rear_sprocket_lock_nuts_#90304GBL003 At £3-77 each, including VAT, they appear to be about the cheapest place for the OME nuts. Fowlers have them in stock as well at £3-86 each, including VAT.
I have looked on Simply Bearings, but they do not appear to stock this type of lock nut.
Whilst the OME are not the same as Aerotight nuts, the design appears to use the same locking principal, but there appears to be little advantage in price. if you used Aerotights, rather than the OME #90304GBL003 nuts, the Aerotights do not have the bottom flange to spread the pressure on the sprocket more widely around the stud.
On no account would I fit nyloc nuts as an alternative, as they will slacken, with the vibration. Alternatives would be to use good quality M10 nuts, drilled across 2 flats and after cleaning and applying a nut lock liquid, torque to the Honda specification and then lock wire all six nuts together. Another method would be to cut paired flat tab washers, as were fitted to Ducati singles and I suspect many other machines.
Personally, I still use the nut lock and paired tab washer on the Ducati and nut lock and Aerotight nuts on the other bikes. Making sure to torque to the OME recommendation, or that given by any good engineering/fastener supplier guide.
I hope this gets you sorted and out and about Joolz, there is nothing more frustrating than being imobile for the want of a couple, or 6, in this case nuts!
Good health, Bill