I think these sleeve or leather bags are good fit for Acer Aspire One.
They are designed for 8.9" Asus Eee. This Acer Aspire One is 8.9" screen too.

9.8" x 6.7" x 1.14" - Acer Aspire One
8.86" x 6.9" x 0.89" - Asus EEE PC 901 (8.9" screen)

Protective Inner Case Bag for 8"/9" Asus Eee UMPC Laptops - $4.99

These bags look good but more expensive:
Protective Leather Carrying Bag for Asus 9" Eee PC UMPC Laptops (white) - $15.76
Protective Leather Carrying Bag for Asus 9" Eee PC UMPC Laptops (Blue) - $14.86
Protective Leather Carrying Bag for Asus 9" Eee PC UMPC Laptops (Black) - $14.84

...but ours came w/ them.

1) DealExtreme stuff are dirt cheap <== 100% true, hardly anyone can beat their price
2) but their stuff are throw aways <== not true at all, they may have some bad/defective items but most of the stuffs I got from them are still good.
3) takes a long time to get your stuff <== they're in HongKong (the other side of the earth), usually take about 1 week to get it
4) bad customer service <== I only have to deal with their CS once and it was not a bad experience.