I've just acquired a Canon HF-G30 and am looking at purchasing extra batteries.

I know people have different opinions on the use of aftermarket "replacement" batteries. For other Canon models that I've owned I've always used "Upstart" branded batteries purchased on EBay through the Canada-based dealer (Bifficus) for Certified Batteries, the North American distributor. I've found the "Upstart" batteries to be very reliable, but I'd never charge them on the camcorder itself, only on the provided charger. And likewise, I wouldn't use any of these third party chargers to charge genuine Canon batteries, even if they are claimed to be compatible. Trashed a Canon BP-808 that way, so lessons learned.

Turns out however that there are no Upstart equivalents for the BP-820 or the 828, so I'm looking for possible alternatives.

Camera battery substitutes sold on Amazon by Big Mike's Electronics, branded "BM", appear to have good customer reviews in general and the supplier is rated as a top Amazon seller. They have both BP-820's and BP-828's but there not many customer reviews for them and, of those, there are just a few commenting on the quality of the batteries themselves.

https://www.amazon.ca/s/ref=nb_sb_noss?url=search-alias%3Delectronics&field-keywords=Mike%27s+Electronics+BP-820&rh=n%3A667823011%2Ck%3AMike%27s+Electronics+BP-820

Anyone have experience with "Big Mike's" BP-820/828's or could recommend an alternative ?

No idea about Big Mike's, but: there have been several past threads on this general topic.
http://www.dvinfo.net/forum/canon-xa-vixia-series-avchd-camcorders/517557-genuine-canon-batteries-used-xa-10-not-compatible-xa-20-a.html is but one.
http://www.dvinfo.net/forum/canon-xa-vixia-series-avchd-camcorders/518532-xa20-third-party-battery.html is another.
Search is your friend.

Thanks Don.
Well, curiosity got the better of me and I've ordered one BP-820 and charger from Big Mike's on Amazon. Any problems and I'll have a look at Wasabi.

I received the BP-820 and charger from Big Mike's.

The battery is just a teeny bit (1mm) "deeper" (i.e. protruding from the back of the camcorder) than the genuine Canon BP-820, which is negligible. It attached to the HF-G30 without any issues, maybe requiring a bit more of a light push to finally click into place, but it didn't have to be forced.

It required a full initial charge-up which completed as expected with the LED indicator on the charger changing from red to green. I didn't time it, but the charging time was considerably shorter than charging the genuine Canon battery on the camcorder.

On attaching the charged battery to the camcorder and powering on, it behaved just like the genuine Canon battery. It is rated at 2000mAh /Voltage: 7.4 volts according to the advertised specifications and the "BM" branded label on the battery itself. With that first full charge-up, the camcorder reported the battery life to be 184 mins in "idle" Camera mode and 270 mins in Media mode. The genuine Canon BP-820, rated at 1700mAh /Voltage: 7.4 volts, shows 182 mins and 267 mins on a full charge, so there's actually not much between them.

Using the camcorder with the BM battery yesterday, the power consumption behavior appears to be no different from the genuine battery - I didn't see any sudden rapid drop in battery life as some have reported with generic batteries.

So the initial indications look good, but I'll continue to monitor the performance of the BM battery and see how it holds up before committing to buy more.

The charger itself came with a car power adapter and a "flat-to-round pin" adapter for the integrated "swing-out" two pin wall plug. Plus a couple of freebies - lens cloth and a cut-to-fit LCD protector.