Well there are literally hundreds of strap makers, prices ranging from 30 bucks up to over 200. Try the supporting vendors they are usually cheaper than the big players equal quality most of the time. The sizes are easy once you know your wrist size if your average, which is around 7" circumference then 130/80 is standard for a 4/5mm thick strap you'll need 24mm wide too. If you wait to get the watch and see how the rubber fits. If you like the size then I would suggest 120/75 3.5mm thick.
The bigger no. Eg. 130 is the long part that passes through the buckle the lower no. Eg. 80 is the short part that the buckle attaches too. The lowest no. Eg. 24 is the width.
There are many different combinations to be had. You could have 24 at the lugs 22 at the buckle(this would fit the tang buckle that comes with the rubber you'll receive) 130 long part 80 short part. The sizes are more often than not measured in mm
The thicker the strap the longer you need it to be to account for the added thickness.
Eg. I have a 7.2" wrist so I would wear 125/75 for 3.5mm thick 125/80 for 4mm thick and 130/80 for 5mm thick.
I hope this helps.