If you're being serious, and not just boomer meme-ing, your best results will be found by reading up on the most capable Gen 1/Gen 2 decks by navigating this site here:
https://
jklaczpokemon.wordpress.com/You can also try searching for 'Old School Pokemon TCG' and watching a few youtube videos on the decks created during whichever era you want to play in. It's been a long time since I last checked, but I believe the channel 'Tricky Gym' should have a few deck tech videos and some modern recordings of matches played using the old decks head-to-head.
Even if you don't want a truly competitive deck are just want to construct some jank lists, still take the time to parse through the info on the site. It will at the very least make you aware of which cards are genuinely good, and which cards are just plain not worth it.
As for purchasing your cards, you're kinda in an awkward time for collection gathering. In the past few years, old school Base Set-era printings of "competitive" cards have shot up in price; whether it's the market drying up or collectors on a nostalgia trip, IDK. Add on the fact that the pandemic has people reconsidering their finances. Some who need some cash right now will be more keen on selling their cards for cheap, but those with a steady income will more than ready to take advantage and buy up those deals as they get listed.
For specific sites for purchasing, I'm not sure where you're located in the world, and that DOES matter in regards to your best buying options. EBay searches for individual 'singles' of cards *should* be consistent regardless of location, but in the US TCGPlayer has done me well enough for YuGiOh, Magic, and Pokemon. There might be a local card shop near you that sells old singles on an online storefront, so give em a call and find out. CardMarket is good if you're in Europe. Check around at some random youtube videos from Magic or Pokemon TCG content creators. Lots of them have 10%+ off deals with specific sites.
