If you need 50 ohm for DC or near-DC applications, you'd be better off making a DIY one. Sometimes it can be useful, for instance, I sometimes use 50 ohm components to deliver a low frequency signal to a 'scope for convenience, rather than 'scope probes. But usually I don't mind a few percent error.
Otherwise, for $20-$30 ballpark the Huber & Suhner ones appear to be a lot more close to 50 ohm at DC than the MiniCircuits parts, but that's just an observation based on a limited sample, and would be easy to improve on with a DIY load, at DC. If you search for 'DIY OSL' or similar, you'll see lots of examples of how those can be assembled. I have a few DIY ones, and personally I measure a dozen or two resistors (2-wire, I don't bother with 4-wire for this use-case), to pick the ones closest to 50 ohm (when combined; usually I'll use two 100 ohm resistors in parallel), simply because it's easy to do.
Even calibration kits (3- or 4-digit $) might not come with any measurement at DC. My fairly decent (but not metrology grade) OSL 'T' cal tool doesn't come with any measurement at all for the 50 ohm load below 50 MHz.