Finding prerecorded Japanese VHS/Betamax tapes?

The sites you mentioned (YJA and Mercari) are the two I recommend. Though, if you search for "VHS" on either of those sites, you'll be swarmed with results. Some advice:

-ビデオテープ tends to provide more blank/prerecorded search results than just VHS. VHSテープ can sometimes come up with different results, and for Betamax tapes, ベータテープ can come up with different results.

-Unlike eBay, sellers on Yahoo Japan Auctions very rarely show the labels on their tapes. Most of the time they'll either be censored with a photo editing software, or they'll be omitted altogether. From my experience (and for some reason), on sites like Mercari you're more likely to find tape lots that have their labels pictured. Keep in mind though that it's not very common.

-Knowing how to read Japanese is not necessary, but is extremely helpful, especially when it comes to reading labels. When I was first starting out, I would use the Google Translate app to scan listing photos to make out any kanji/vocab that I couldn't understand. This can be hit or miss, as some labels are not clearly written.

-If you're fine with purchasing tapes with unknown material on them, be prepared for anything. The first VHS lot I purchased was completely random, and there were adult movies mixed in lmao

-Some sellers will say that their tapes are from a specific time frame, such as the 1980s or the 1990s. These are usually legit claims, though be cautious if a seller claims they have something specific (that is not otherwise stated on the label). It's very common for sellers to say that they have a specific anime/drama series in their descriptions, but these are often scams.

The same rules for purchasing used tapes on eBay also apply here. Watch out for tape mold. Just because something is on a tape label, doesn't mean it's still on the tape. Bulk lots are heavy and hard to ship out of the country, so start with very small lots of around five or so. (In general/my experience) Betamax is more likely to have material from around the 1980s.