I simply searched for "feed-through terminator 50 ohm" at eBay 
This model is obvioulsy available from several sellers, and at different prices.EDIT: Retracted answer - the question did refer to a different model.
The 50 Ohm resistor of the feed-through terminator is not placed in series with the input of the scope, it's in parallel.
There is, however, the 50 Ohm source impedance in series, but agreed, the low-pass is eventually formed by only 25 Ohm and Cin, doubling the cut-off frequency.
Exactly, the cable impedance and 50 ohm termination mean that the oscilloscope input is driven by the parallel combination of 25 ohms and that combined with the lumped input capacitance produces an upper limit on the input bandwidth which is why higher bandwidth high impedance inputs have lower input capacitance; some are below 10 picofarads.
Incidentally, the same applies to how oscilloscope probes are specified including active probes. A special probe tip adapter is used which has the feedthrough termination at the probe tip so the probe is also driven by a 25 ohm source. The probe delivers less bandwidth when attached to an impedance greater than 25 ohms.
...that the oscilloscope input is driven by the parallel combination of 25 ohms and that combined with the lumped input capacitance produces an upper limit on the input bandwidth...
So the obvious step to the next level would be to terminate the cable at the scope end not with 50 Ohm, but with a (say) 40:10 Ohm or 45:5 Ohm divider, reducing the impedance seen by the scope input even further to < 10 or < 5 Ohm
Overkill?
So the obvious step to the next level would be to terminate the cable at the scope end not with 50 Ohm, but with a (say) 40:10 Ohm or 45:5 Ohm divider, reducing the impedance seen by the scope input even further to < 10 or < 5 Ohm Overkill?
That will work and some probes effectively do that but most of the attenuation will be at the probe end to reduce the capacitance from the probe's cable.
I got these ones for just a hand-full of bucks:
https://www.ebay.com/itm/50-Ohm-Feed-Through-Terminator-BNC-to-BNC-50KY-Device-Q9-Adapter-Black/312248423345
Seem to work pretty well up to the BW limit of my scope.
Would be interesting how well they work up to the specified 1GHz.
I can't measure that, though, with my equipment.
I bought a couple of those inexpensive P57 50-Ohm-Feed-Through-Terminators from eBay. They work better than a T with a 50 ohm terminator. AT DC they measure exactly 50 ohms and when I measured the return loss it was at least 25 dB (SWR 1.1 or less) up to 170 MHz. The BNC connectors have good contacts and the center pin is the same diameter as my scope probes.
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I just got a VNA so I re-tested the P57 50-Ohm-Feed-Through-Terminators in my post above. I was quite surprised at how good the return loss was up to 750 MHz. RL better than 25 dB (VSWR < 1.1) at all frequencies.
Hello there
I have all of these termination types.je, mostly the Tektronix, EGG, and HP.
The Chinese knockoffs are poor quality mechanical in the connectors.
I very highly recommend the Mini-circuits parts for terms or attenuators
Very nice service on small orders.
Paid $10-15 each.
VERY well specified and VSWR ~ 1.2 up to 1-2 GHz
Just the ramblings of an old retired EE !
Jon
A few weeks ago, I´ve bought my own 50Ohm Terminator due to a auction on eBay - A Huber&Suhner 6701.01.B, with about 120 bucks not really cheap ( got it for 25...
).
The overshoots using this one instead the former terminator from work are signifiant less (but of course still avaible).
...
I very highly recommend the Mini-circuits parts for terms or attenuators
...
Is it possible to provide a reference for a feed trough 50
BNC terminator (similar to the Rigol P57)?
Appreciated
I've been there, but was unable to find a BNC feedthroug 50 terminator. Only the "regular" ones to use with the T's.
Or could be the case I'm not looking hard enought 
I could not find any either. I do not think Mini-circuits makes feedthrough terminators.