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Bus drivers in Macau don't accept Octopus/credit/debit card, don't give any change back, and bus tickets typically cost 6 MOP. Additionally, if one pays with HKD, the exchange rate is disadvantageous (1 HKD = 1 MOP instead of 1 HKD = 1.03 MOP). Therefore one must pay bus rides in MOP coins if one wishes not to overpay (unless one has a Macau bus card).
Where can I get MOP coins in Hong Kong?
I think the best chance you can get any Macanese pataca (MOP) coin is in Macau, since in Hong Kong the only legal tender is Hong Kong Dollar (HKD). But even in Macau it really depend on your luck if you'll even come across MOP coins. I've traveled to Macau multiple times and rarely got MOP as changes. So if you're in Hong Kong I think save some HKD coins for bus in Macau is the best solution.
But I see in your question you want to save the difference in exchange rate when travelling on bus in Macau, so you'll save around HKD/MOP 0.2 per trip. To make this saving significant enough I believe you'll need to travel on bus a lot, and if you are planning to travel a lot on bus you might well consider buying a Macau Pass (the site seems to only have Chinese version).
Macau bus fare is normally MOP 6, and if you use Macau Pass to travel, you'll have MOP 2~3 discount (depend if it's express route, which has alphabet 'X' in the route number). A new Macau Pass cost MOP 130 (MOP 30 deposit + MOP 100 top-up). You can get the stored value and deposit back by returning the card, but if you return the card within 2 months of purchasing the card, they'll charge you MOP 10 handling fee.
Put it into calculation, if you travel for more than 4 trips on normal bus route (6 trips for express route), you've already taken advantages of Macau Pass.
And you can buy your Macau Pass with MOP notes, so no need to worry about preparing MOP coins.
