Supported rail - a couple of lengths of any size for an experiment!

I'm looking to source a couple of lengths of supported rail for an experiment I have in mind. Just trying to keep the spend within limits, because the plan might not work, hence casting around for a used pair. I know quite a few builders on the forum have in the past upgraded their unsupported rail to profile rail, and so wondered if anyone had some redundant rails kicking around!

I'm not fussy as to what size rail it is - ideally 16mm or more, but even 12mm would do it! Length has to be at least 750mm, but I'm happy to trim accordingly. And if it comes with any bearing blocks, then all the better - but not essential. I only really need one per rail for what I have in mind, so purchasing these new wouldn't break the bank.

Thanks in advance!
Graeme