Try these classic triangular baseboard registers.  New and improved but won't put a dent in your wallet. 15", 18" and 24" sizes available here.

Purchase new baseboard registers for your old, worn out, or damaged registers. 10" x 6" and 12" x 6" sizes.

Antique brass wicker baseboard registers by accord.  15" and 18" sizes available.

Hard to find 4 foot Metal Baseboard Register in traditional white or golden sand (light brown). Great for any commercial or residential application. 

Brass plated baseboard diffusers look much better than plain white registers; available in 18" lengths.