Your image has an aspect ratio of 1.77:1
Most prints sizes are either 1.33:1 (e.g. 8x6) or 1.5:1 (e.g.12x8).
You won't find many services that print 1.77:1 (although it does correspond to 16:9 which some cameras do shoot)
One option is to order a 1.5:1 size and select the option to resize to fit rather than crop when ordering which will give you a print with a white border top and bottom that you can then trim off.
regards
Brian