Hey, I missed your comment. Nowadays the best source for optics is often a secondhand instrument where you scavenge the optics. If we specifically talk about a 6"f/5, most recent industrial tubes have really decent optics.

Buying new optics online, avoid any ad where the price seems surprisingly low. For example you can find 6"f/5 mirrors advertised as lambda/8 precision. There's a lot of abuse with this denomination : some sellers use surface precision, whereas we usually talk about reflected wavefront, so lambda/8 becomes lambda/4.

Then, there are a lot of ads for optics at great prices that aren't specified as parabolic (in the case of newtonian mirrors). Most often those are spherical, which is totally unsuitable except for longer focal ratios (and that quickly becomes impossible as diameter grows).

On the other hand, I recently ordered a 40$ "lambda/8" spherical mirror, and it was a great sphere, so a great starting point to polish it to a parabola.