If I understand you question correctly, you are currently using the Power Functions M motors such as this one:
I did a quick search on eBay, and most of the listings I see are not genuine LEGO PF M motors. They are listed as "for LEGO", or LEGO compatible, but they are not original LEGO products. Here's an example of one of the third-party motors:
It looks very similar, but has many subtle differences. The easiest difference to spot is the lack of the word "LEGO" on the studs.
These may be good quality, or they may be significantly worse than LEGO products, but in any case, they were not manufactured by LEGO and did not go through TLGs QA process. These products also likely violate TLG's IP in some way.
The reason that it is difficult to find genuine PF M motors is because the entire PF line has been discontinued and replaced by Control+ in Technic sets. The closest replacement for the PF M motor is what TLG currently calls the Simple Medium Linear Motor:
Unfortunately, this motor is also retired as a standalone product on LEGO.com, although it was included in sets as recently as 2022.
If you are looking to something that you can buy today directly from LEGO.com, the Technic Large Motor is probably the best choice.
Transitioning to the Control+ system would also require purchasing new battery boxes, which are not especially cheap.
One approach is to purchase genuine PF medium motors on a secondary market such as Bricklink. Here's the listing for the PF M motor there. Most established Bricklink sellers take quality very seriously. They know how to identify non-LEGO products and will work hard to make sure you only get genuine parts.