Wanted to correct myself and say that in addition to T440p, the W540 and W541 also have socketed CPUs. However, they're becoming memepads which has seen their prices go up considerably.
summary: Classic Thinkpads (X200->X230, T400->T430, X60/61, T60/61) are becoming less budget-friendly, especially when you factor in the cost of repair (may be necessary) and maxing out the specs for modern use. If you want something to handle modern workloads and still want maximum upgradeability, then T450/T450s -> T480/T480s are your best bet.
As for buying them, avoid big retailers like Best Buy and Amazon. Ebay is usually the best secondhand market, but you can also find plenty of gems on shopgoodwill and craigslist