It seems like the Nagatanien (永谷園, a Japanese food manufacturing company) Variety Stickers are from an ochazuke (お茶漬け) Pokemon-themed meal bag, from sometime in 1998-2000(?).
Here’s some advertisements that show the stickers and the meals:
From 1998 (stickers at 0:11) https://www.youtube.com/watch?v=F2yBAai8e8s. (There’s more advertisements on YouTube from 1999 and 2000 if you search up “永谷園 お茶漬け ポケモン”),
Nagatanien’s website also dates the Pokemon-themed furikake releasing in 1998: 沿革｜会社情報｜味ひとすじ 永谷園 Furikake is similar, but not quite ochazuke, however. Also, from pictures online, it looks the stickers would fit perfectly inside the ochazuke bags, but not the furikake boxes.
There’s also a news report dated from 1998 on the growth of the company due to the Pokemon tie-in, which mentions the ochazuke: 永谷園の「ふりかけ、お茶漬け」が好調、「ポケモン」群が寄与 - 日本食糧新聞・電子版
I didn’t see the specific stickers you had, but it looks to be from the same series. Other Nagatanien items are likely to also be distributed as an extra bonus with food.