From Australia To The World

One of your best bets would be to attend the Petersham Coin Show, at Petersham Town Hall Crystal Street, Petersham (Sydney). Happens 4 times per year (every 3 months) on the 5th Sunday in the month, where there are 5 Sundays in that month.

Usually about a dozen specialist coin dealers in attendance.

The next dates for the Petersham Coin Show are:-
Sunday 29th August 2021
Sunday 31st October 2021
Sunday 30th January 2022.

I have attended every show for about the last 6 or 7 years.

For me, when it comes to World coins, I collect all silver and base metal coins before about 1950, right back into ancient times.
I am a regular searcher in medium and low priced dealers' junk boxes, as well as higher priced individual coins. Have been doing this for nearly 50 years.

Sometimes, I buy job lots of World coins from auctions but I tend to be rather 'picky' about which lot(s) I may bid for, and I usually leave low fixed priced written proxy bids with the auctioneer, expecting to be unsuccessful.
Occasionally, you get lucky with a decent bargain, and end up with a job lot with few duplicates against the rest of your established collection. In this sort of situation, it is best to examine the contents of the lot personally at the auction view day(s), mainly to avoid duplication.
I have done this with Noble Numismatics and I.S. Wright Auctions.

The best way to do it is find a few different sources (eg. auction houses, coin stores, different sellers on ebay/gumtree/tradingpost. The first time you buy from each seller, create a spreadsheet listing your valuable finds and which seller you got them from, then compare it against the price they charge per kg (inc postage).

My two fav sources charge $170 per 10kg (average results), and $279 per 10kg (excellent results. I always end up making a profit from these sellers. Other sources have fairly mixed results.

If using ebay or Scumtree, do some digging in the sellers feedback or other listed items. If they are selling or have sold lots of individual lowish value coins then it's a sure bet they have already noodled the batch you are about to buy and you are purchasing nothing more than scrap metal. I admittedly do on-sell my noodled coins in bulk however I only remove coins that I either don't have, am upgrading or are using as trade with someone else. I do regularly put nice coins back in the bucket as opposed to someone removing every coin of slight value and flogging it. Treat terms like "deceased estate" with a pinch of salt, especially when none of the other items from the seller look they came from an estate. I once saw a lady selling on GT a bunch of old Oz coins which she described as unsearched and "may contain hidden treasures" yet in the background of the photo was an exercise book with a list of shillings, sixpences etc that she had obviously pulled from the pile and how much she thought they were worth. Priceless.

Tardingpost? I thought it had gone the way of the DoDo.

Like drowning said auction houses this is where a lot of estate sales are normally sold off in.

Keep an eye on Police auctions of lost and confiscated items. I know in Perth there are always bags of foreign coins