There are a series of bike shops across the street from Western Canned Foods (66 Ba Trieu).
I don't have a map with me, and I happen to be out of the country at the moment, but there is also a series of bike shops in another area (and many more I don't know about ;^). If you are by the Temple of Literature at the intersection of the main road that comes into Nguyen Thai Hoc at the light - if you go out the main road, (south WEST?) past the Temple of Literature a couple of kilometers there is a big intersection with lots of bike shops. I used to go out that way by bus (several times) to an FPT computer shop and FPT office block when I was getting my ADSL hooked up and noticed the bicycle shops at one of the big intersections - many more than across from Western Canned Foods. Sorry I can't give you a street name - if I had a map ..... I can not vouch for any of the shops anywhere.
If you want an imported bike check the bulletin boards at L's, Western Canned Foods, etc.
Thanks, ldldjr, for your pointer - they didn't have the one I wanted so I took a day bus across to NanNing where they had bikes that cost upto USD$1000+ and all sorts of spares (kevlar tire guards) at OK prices.