I had to retire my '81 peugeot UO9 this year. I don't know anything about the new parts market for singlespeed other than volumebikes, se racing, and other bmx brands; i always dug up used stuff. But I'm looking for either a frame (that fits my fork/stem) or a frameset with fork/stem (700c), for around $400 or lower.

I really messed up when I bought my bmx awhile back, I didn't ask a forum first and ended up with an overpriced lemon.

This time I want a bike that'll stick it out;

suggestions on what to look for either new or used online, and links, would be really appreciated.

what kind of riding do you do?

do you want steel or aluminum? at that price range, you can get a lot. either a complete kilo tt (or other bikesdirect.com complete), a kilo tt frameset, leader frame, used fuji track pro/cannondale track/gt gtb/other older track frame, and the list goes on.

figure out what you want, then check craigslist and ebay.

hell you can even get a new 2010 fuji track pro frameset from citygrounds.com for $450

On thebikebiz they have some soma frames with fork for under $400. They are Tange Prestige tubing.

more or less just commuting in montreal. So, kind of rough riding but no tricking and no track; well maybe track if possible, haven't been in years though

How rough? Everyone will tell you to get a Kilo, but if you want more tire clearance an alternate Kilo or Steamroller might be better.
There's a bunch of bikes out there for $400 or lower, basically all the same material (average to good steel - TANGE!)
If you want aluminum Leader sells cheap frames, same ebay random framesets (VISP)
Hope I helped!

Bonjour, je suis de Vancouver.

Well, montreal roads are awful, and i curbhop like a mofo

and as a bmxbro, probably go with steel