eBay eBay eBay...
That's all I can say.
I got me the U. B&B 6 & 7 there, it's really worth the money,
but if you wanna get something in the likes -without the eBay-trouble and sometimes long periods of searching-
I'd suggest you the Prosamples Vol. 24 Breakbeat, Planet of the Breaks. You can get it at Best Service and don't worry, it also is produced by Zero-G (seems Liam is a fan of them, I can absolutely second that).
Ouh, the Megabass 1 is hard to find and trust me, it isn't that good. It was one of the first Zero-G sampler to be released. It wasn't that bad then, but nowadays, since we have things like "Chemical Beats Vol. 1" (Killer...) it's not worth buying that thing for much money.
Breakbeat-Sample-CD's mostly follow the simple rule "The newer the better".
Or, just sample the Ultramagnetic's and give them a little bit more pressure.