Great boots! And I agree with Alice to check eBay current and sold listings. When I have boots, I almost always list them on eBay, and there are always good "comparables" to look at for pricing and even, sometimes, dating help. I'm thinking yours may be from the 80s, with that blue shaft. Although they probably made (still make?) colored shafts on their boots, the toe shape looks older than contemporary, but I'm not sure....
There are also a couple of sites that specialize in vintage & used boots--you may be able to find a pair like yours on those sites.