If the signal is single-ended and ground referenced, then you don't need a differential probe and you can probably use any ordinary FET probe. These typically have an input impedance of 100 kOhm to 1 MOhm in parallel with 1 to 2 pF capacitance, and have a typical bandwidth around 1 GHz. Of course a few very old or very cheap ones have less bandwidth, and the most expensive modern ones can go much higher. You should be able to get a used working one for $100-$250 USD. Note that all FET probes need a power supply, and the Tek ones usually require either a Tek scope with TekProbe connections or an external power supply such as Tek 1103. I use one of those with my Lecroy scope and Tek P6243 probe. Likwise, Lecroy active probes need a matching scope connection, and I haven't ever seen an external supply for those. Used Lecroy probes usually cost more than comparable Tek ones, but used Lecroy scopes themselves are usually a bargain.
If you can't measure the signal w.r.t. ground then you will need a differential active probe, which will cost much more. You still need to think about the power supply.