To make it a bit more of a story of my non-TCG Seviper collecting, while at the same time answering your questions in between:
At the start, before I was actually started seriously collecting non-TCG Seviper stuff in the first place, I just randomly bought cheap stuff I came across. Here is a picture from August 2020 for example, with all the stuff I had randomly bought over 2-3 years. I was also only focusing on stickers and cards; basically things I could put in a binder:
At the time when this picture was taken, I had just added the Japanese unlimited edition WCP Battle Frontier TCG card with Seviper cameo to my collection, and my TCG Seviper collection was only missing a single card: the Seviper from the same Japanese unlimited edition set, which I added a few months later in October 2020. Because my Seviper TCG collection was then 100% complete, I decided to delve deeper into the non-TCG part, and it’s actually a lot of fun to do.
Just like with most of my other collections, I just started buying things I could find. Mostly cards and stickers still, but also both plushies, most of the figures, etc.
I also googled for other people’s Seviper collections, but as you can imagine, there aren’t too many of us who collect Seviper specifically. There is however one big collector that I mostly used as reference, which is Marphoria. I came across her SnakeGhosty website first, and used that as reference for all those Seviper items. I later on also came across some of her update posts on PkmnCollectors.LiveJournal, and I actually saw last week she posted her collection on IG back in 2021. Arbok is her main collecting interest, but she collects non-TCG for quite a few Pokémon, and Seviper is one of them.
Later on, I also came across the MyFigureCollection Database Seviper page, and this Japanese Seviper Fun Club, which I’ve used as additional references.
In addition, I also went searching for Seviper in all kind of non-TCG wiki pages, like PkmnCollectors.Fandom; Bulbapedia Merchandise and cards; this VS Cards & Amada stickers website; etc. Especially that first one gave me quite a few new items to look for.
And over the years I simply added to the collection like this. I also came across NidoKingdom on IG (also a member here on the forum since this year), who helped me with quite a few items as well (1/3rd of the Amada stickers came from him iirc, but also one of the binders and the Meiji quartet sticker to give some other random examples).
And dozen of other collectors also link me auctions of items they think I might not have yet or they’ve seen in my wanted list, which is also very nice.
Anyway, as a kid I only ever collected TCG. And even when I came back I’ve collected TCG only for about the first five years. But now, I defintely don’t regret delving into the non-TCG side. There is so many unique items to find, and it’s also fun searching info about them, or even discovering them yourself.
Take the Kraks for example. I’ve seen Seviper in four of the colors very often: transparent, yellow, neon-yellow, and lilac. And orange is slightly harder to find, but also not too uncommon. But the other ten available colors I’ve only seen 0-3 times each for Seviper, even though they’re available in abundance for some of the other Pokémon. For a very long time, I was convinced certain Pokémon were only available in certain arrange of colors. But later on I discovered, and I’m not entirely sure why, that the Kraks color distribution per Pokémon is just extremely uneven. Still, now that I’ve found 12 of the 15 colors for Seviper, and I’ve seen one of the three colors I’m missing once, I now know all fifteen of them are available for every Pokémon (I can’t even imagine someone going for the full set of every Pokémon in every color 
).
But then don’t even get me started on the second set for which I have that brown-greyish Seviper, for which I basically know next to nothing. Can’t find any info for it, although I did manage to buy a booster pack that I’ve opened and contained three random gen 3 Pokémon with different colors and holo patterns. I certainly hope I won’t have to find the Seviper in not only 10+ more colors but also different holo patterns in a set that’s almost impossible to find in general, haha. 
Anyway, I’m digressing.
Lots to search; lots to collect; lots to learn; lots to document. 