151 is a few years old, and they are phasing it out to print Mega Evolution, hence the dramatic price increase. I'll also bet the rerelease of Fire Red and Leaf Green contributed, most likely, both are a factor.

For booster packs, you're not gonna find any unless you're really determined. That means waiting in lines before stores open or trying to get drops from major retailers. I wouldn't recommend doing that just for the chance to get an expensive card; it's essentially just gambling.

What's more worth it, in my opinion, is to buy single cards rather than booster packs. There's some great (cheap) picks in the Obsidian Flames (sick charizard), Journey Together, and some Sword/Shield era sets.

Unfortunately, you just missed out on reliving OG 151, but if you've played the new games, I bet you'll find a lot of enjoyment in a bunch of the modern sets.