https://www.retrostic.com/roms
this is the link ive had the best luck with. most of the ROMs work as is. just put the title of the game your looking for in the seach bar and it will give you the results for that game for all platforms. ie. if you put in donkey it will bring up all the games that have donkey in it, which all the donkey kong games will come up. it will show the platform that rom is for and number of downloads. i just scroll down the list and looked for MAME and download the one with the most downloads. its normally the first mame entry.
https://archive.org/search?query=mame+rom
this is another good site. didnt have much luck with the search but found this page and use it to go to each list by alphabet letter.
when you launch the game and it says there a few files missing, write those down and do a search for them. most of the time Internet Archive will have them. just download it and add it to the zip file. for Namco games go get these files and just add it the games folder. Namco ROMs: Namco50, namco51, namco52, namco53, namco54 and namco62. namco games will pull what they need from those folders.
some of the star wars flight games you will need quite a few additional files. again just google them. there is a site (https://www.legeek.info/2019/09/20/mame-rom-update-0-212-to-0-213/) that will have the missing files in one file. just download the file and take out what you need.
for example if your looking for the file 315-5572.bin, google it. one entry will be for legeek.info. that link will take you to a page that has all kinds of files on it. do a find for 315- and you will find the file you need. it will show this \swa\315-5571.bin. that means that file is in the SWA file. so when you go to download you download the SWA file. the 315-5571.bin will be in there. just copy and paste it in the rom thats asking for it. it will make sense once you do it.
the one file i have not been able to find is dvctbd.bin. i see it but havent been able to get to it to download. if you find it let me know.
any questions let me know.