Seems like most of the replies have no clue.
Try:
https://www.apmex.com/?gclid=EAIaIQobChMI8P_rqb_B3wIVGHZeCh0fpQasEAAYASA AEgJKlfD_BwE
I have always had good service with APMEX. Even the US Mint is a source of coins.
https://www.usmint.gov/
If you are looking to buy old gold coins, even eBay can help. Or coin shops. I got a complete set of 1903 gold coins off eBay a couple years ago. All in MS64 condition too. A 20$, 10$, 5$, 2.5$ and a 1$ commemorative. I got the 1903 series 'cause that was the year my dad was born. I am still working on the 1920 series for my mothers birth year. I even picked up a 1795 offset bust silver dollar off eBay for 3K but is worth around 8k today.
Find out what years your wife's relatives birth years are and get those years. May take a bit of looking, but they are out there. Except 1933. That is when the fed stopped the gold standard and got everyone to turn in their gold for silver. Not sure when the gold coins were once again minted.
Also, go here to look up values:
https://www.pcgs.com/
If you are interested in gold bars, APMEX is a good place to look. Take a look at the PAMP gold. Top quality Swiss gold of which you will pay a premium. They even have a 1 kilo gold bar for $41,944.43. Paint it black and use it for a door stop.