If you're building a PC inside a small form factor case, you're going to need to compact hardware to go with it. That typically entails either a micro-ATX or mini-ITX motherboard, and sometimes a smaller power supply as well, depending on the specific case. There are plenty of shortened PSU options out there, like Corsair's SF600. It's a 600W unit built to SFX specifications, and is on sale at Newegg for $94.99 right now.
You could also wait for Silverstone's SX1000 Platinum to be in stock if you're shooting for the moon, as it's the only 1,000W unit around in an SFX-class form factor (SFX-L, which is slightly longer). It will undoubtedly debut for more than twice as much, though. Meanwhile, Corsair's SF600 is a high efficiency unit that is 80 Plus Gold certified.
Corsair SF Series 600W PSU | 80 Plus Gold | Fully Modular |
$124.99 $94.99 at Newegg (save $30)
Built for compact cases, this 600W power supply is 80 Plus Gold certified, meaning it runs efficient and wastes less energy as heat, an especially important factor when building a PC in tight confines. with limited cooling potential.View Deal
There's enough wattage here to power a modestly powerful PC with up to a GeForce RTX 3060 Ti, and perhaps even higher (Nvidia recommends a 650W unit to drive its GeForce RTX 3070, based on a PC configured with an Intel Core i9 10900K, noting that "a lower power rating may work depending on system configuration"). Likewise, AMD recommends a 650W unit for its Radeon RX 6800.
This is a fully modular PSU, so even the main 20+4 ATX connector is removable. You can't run a PC without that cable, but being able to take it out can help with manoeuvring this PSU into tight confines.
One other thing worth pointing out is the warranty—Corsair guarantees the SF600 for seven years.