Its $29.00 for a large before shipping charges. And I bet you could get it in the John Deere pattern. I don't know how big around Walkers girth is but he should fit in a large. Here is the chart they have.
HARNESSES
Size Girth Width
XS 10-14"
1/2"
S 14-20"
1/2"
M 20-28"
3/4"
L 28-36"
1"