You can buy the TBS cards direct from the TBS store at TBS6925 DVB-S2 TV Tuner Card PCIe but sometimes they are also available from a seller on eBay. However you should be aware that is an older card that is no longer supported by TBS, they now recommend you purchase a TBS6983 instead. Personally I would stick with a TBS card; when I first got into trying to set up a backend system some folks on another forum (that I should have known better than to listen to) were really pushing the Prof cards hard, probably because the guy that ran the board was selling them. So I bought one but never could get it to work completely right, whereas I've had four or five TBS cards and none gave me the problems that Prof card did. It's been so long that I don't recall the specifics but it seems to me that part of the problem was it used the older PCI type edge connector which I do not think is compatible with modern PCI slots, but the thing I do remember is that you could not have more than one of them in a system, apparently it never occurred to whoever designed the stupid things that anyone might want to use more than one and IIRC it would just ignore the second one. Whereas I currently now have four TBS cards in one system (see my avatar pic) and all work fine.
This is not to say that the TBS cards don't have their own issues, at least if you are using them in a Linux backend. In Windows they may work great with no special intervention but if you are using Linux (to run Tvheadend, for example) there are a few tricks. One of my biggest issues was resolved when I found this article about interrupts and applied the suggested fix - up to that point, the system worked pretty well but every so often the card would appear to stop working until the system was rebooted - this has not been an issue since applying the fix shown there. The other issue in Linux is that you have to build the tuner drivers, they are not included in the Linux kernel! And if you are not fully steeped in the way of the penguin, that brings its own set of challenges, particularly if you do get the older card that's not supported by their current drivers, which don't appear to support the TBS6925. The thing I most hate about the TBS drivers is that every time you install a Linux kernel update (which seem to come along every couple of weeks in Ubuntu) you have to rebuild the drivers again. So if you are more familiar with Windows, or your knowledge of Linux is not that great, I'd stick with Windows and (assuming you want to be able to record signals) maybe use Windows-based PVR software such as MediaPortal or NextPVR - I am not recommending either of those, they just happen to be two that I know of that work in Windows. I do know that MediaPortal has at least some support for satellite tuner cards, not sure at all about NextPVR. However if you do know Linux and aren't put off by needing to rebuild drivers, then I'd at least take a look at Tvheadend, that was the one that made the most sense to me although YMMV.
I will just say be real careful about buying older stock that's no longer supported or that may have issues that have been fixed in newer versions. There might still be dealers that I think overbought on certain cards and are still pushing them even though they are way out of date and may not even work with modern computer motherboards. If you know what you are doing they possibly can still be made to work, but doing so typically requires a lot of specialized knowledge (especially if used with Linux), and the people who already have that knowledge may not be particularly interested in sharing it.