Switches: when I last looked, there were cheap mikrotik switches and tp-link, if you want to buy new. Otherwise search on eBay/craigslist/etc.

Network cards: intel, mellanox, solarflare. Intel has a lot of drivers for all OSes, mellanox is most stable electrically (you can provide it with separate power, no need for fancy adapters, just plug any 12v in), solarflare is cheapest on ebay if you buy 2-3-generations old cards. Actually, any 10G network card will do, just check if they have drivers for your OS.

Thunderbolt: there are ready-made eGPU cases, they are costly but easier to handle. If you want to be cheap: wait for sale of any thunderbolt3-to-nvme adapter. PCBs for all of them are made by the same factory (you can find it on alibaba/taobao) they differ only by box and branding. Based on intel JHL6340

edit: I have got this, it was on-sale at the time https://www.aliexpress.com/item/4000200151507.html

edit2: just checked eBay, the cheapest solarflare "S7120" (it is SFN7022F actually) is $15 (!) + free shipping, wow. Though you'll need some sfps, they start from $6 (FTLX8571)