I’ve been looking around for substitute button plugs (they’re quite expensive for what they are) and eventually scored these: http://www.ebay.co.uk/itm/HP-30-Nylon-Locking-Hole-Plug-Button-Cover-30mm-1-3-16-7DS-x-20-pcs-/190827169709?pt=LH_DefaultDomain_0&hash=item2c6e2f87ad
I received these today, and can happily confirm they are almost 1:1 with my Sanwa OBSM-30s. They are slightly thicker, and have a marginally different texture, but for all intents and purposes are the same.
If anyone is interested in buying enough plugs for the next 10 arcade sticks they buy (20 plugs), for £4.50/$7 shipped, I highly recommend these. Postage from Hong Kong to UK took 6 days.
I gave a couple of these to gahrling (grappler scrub), so he may confirm my appraisal if needed.