You can try Tran Anh computer, I think they have them. It's one of the best companies in Hanoi.
Addresses:
HQ: #1174 - Lang road - Dong Da dist.
Tel: (04) 7.666.666; Fax: (04) 7.667708
IT equipments supermarket: #292 Tây Sơn st.- Dong Da dist.
Tel: (04) 537. 6666; Fax: (04) 537.5324

I use and recomend Mr Nyuyen Xuan Hau, 0913514744 He provides personalised repair services, as well as procurement. He was listed in the latest Hanoi Guide.

Depending on the brand and model of your desired monitor, it may or may not be available.

I recently obtained a Samsung SyncMaster 2253BW monitor (22", 2ms, 8000:1)It took several tries and about 10 days to get as it had to come up from HCMC and there are not many of them around.

Tran Anh have their own website and list quite a lot of stuff, but don't always have stuff in stock.

You can try Mai Hoang, reliable computer hardware provider
http://www.maihoang.com.vn/Products1024.aspx?type=8


1
ASUS LCD MK241H
670 11,122,000 36
2
Dell Ultrasharp 2407WFP-HC
944 15,670,400 24
3
Acer LCD Monitor 24" Wide TFT (P243Waid)
467 7,752,200 24
Trang: 1 / 1

Well I second Virezo's suggestion. Tran Anh has a big IT equipments supermarket on Tay Son street, which you can drop by and take a look anytime. I'm sure you'll find what you need in there.
But just to save time and energy, try browsing the products at their website first. Here's a link directly to the LCD monitor category: http://trananh.vn/maytinh/?a=CATE&hdn_category=4&hdn_category_child=28&hdn_category_child_child=215

Thanks for the responses everyone! Maytinh PT (maytinhpt.com)at 89A- Tho Nhuom, Hoan Kiem, sells used 24 inch Dell Monitors in varying conditions and for reasonable prices. Very nice folks there as well, helpful. Trung Anh at 76 Nguyen Du has new 24" BenQ or Asus monitors in stock.