If you know anyone with random keyboards around, try a few out.
I have a Razer Tarantula but it really is a shitty keyboard. It can be pretty quick, its buttons are more responsive than any other controller I’ve tested (generic arcade stick, a couple of HORI sticks, agetec with adapter, 360 pad, and a few others). But over time the buttons can change their consistency and if you’re really unlucky it’ll just be straight up unusable out of the box.
For playing fighters I use an old IBM keyboard that came with an engineering comp in the early 90s. There are situations where I can only press 2 buttons at once but when I use WASD for directions and UIOJKL for attacks I can press any of these: 3p, 3k, LP+LK, MP+MK, HP+HK - while holding 2 direction keys.
I also have a logitech playstation 2 USB keyboard that I bought for $5. It has laptop style buttons and is great for fighters depending on if you can set up the buttons well to work around its limitations.
The problem with older keyboards is you run a greater risk of ghosting. Ghosting is when you press 2 or 3 keys and an extra key you aren’t actually pressing suddenly appears to be pressed on its own. Newer cheap keyboards have anti-ghosting stuff built in, but that often prevents you from pressing too many keys at once.
For a fighter, your best bet is find a nice mechanical keyboard with n-key rollover. I’ve been looking at the SteelSeries 7g myself. It seems like the best bet right now. But its pricey.
However you could strike it lucky if you just check out random keyboards. Many many keyboards can block input once 2 buttons are pressed, but the keys you have to press to cause that can vary quite a bit.