You generally can sell cemetery spaces to any person who wants to buy them. In some cases, the cemetery where you own the spaces may be interested in repurchasing them from you and your brother.
With 104 spaces, you would likely sell to several individuals as families today don’t need more than 2 or 4 spaces. You would need to be able to prove that you and your brother have the deed to the spaces in order to complete a transfer of ownership of a space to another person (this you would handle with the owner/operator of the cemetery at the time you have a buyer for the spaces. The cemetery will not need to know what price you sold them for, they just need to do the paperwork to transfer ownership and there is usually a fee of around $50 for that.) You can advertise anywhere you would advertise to sell something…local newspaper, craigslist, etc.
I would also suggest that you call the cemetery and ask if they would be interested in repurchasing some or all of them. A cemetery typically will not offer to pay more than what was originally paid for the spaces, but this may be a quick and easy way to get them off your hands.