Ordinary laces are as OK as leather laces so you don't have to change 'em. Laces weren't a new invention!
I think it is said the flat rayon sort were more common rather than round but meh.
Still, plenty of laces on Ebay and such
http://cgi.ebay.co.uk/1-Pr-Square-Brown-Leather-Boot-Shoe-Laces-120cm-Long-/120686055686?pt=UK_Crafts_Leathercraft_LE&hash=item1c19731106
http://cgi.ebay.co.uk/AMMO-BOOT-LEATHER-LACES-brogue-service-shoe-army-polish-/280640704042?pt=UK_Clothing_ShoeCare_Shoe_Care_LE&hash=item41577d4e2a
http://cgi.ebay.co.uk/SQ-BLACK-FINEST-LEATHER-3-5mm-HIKING-BOOT-LACES-160cm-/130440125468?pt=UK_Clothing_ShoeCare_Shoe_Care_LE&hash=item1e5ed65c1c
etc etc!
I do confess, rather illogically, that leather laces do feel more 'right' (even though they aren't necessarily) hence my Brit and German boots have leather laces! In fact you have reminded me I must buy some more to replace the horrid thick leather laces that came with my Indian-made Brit boots.