Maybe take her down to the tack shop before hand, have her try some on, but don't really seem to keen on buying them, and then go back a day or so later and buy them when she's not there?
Better yet, get one of her friends in on it, and get her friend to take her to the tack shop and try on boots with her instead. That way your wife will have no idea that you're actually going to buy them. She's just think it was a day out with her friend.
Uhm...My parents bought me tack for Christmas one year, but instead of them picking it out, they got a picture of it(for you, you'd get a picture of riding boots) and they wrapped that up in a box, and the next day the tack shop was open, they took me there and I got to pick it out instead of them. It was nice being able to pick it out myself.
If I come up with anything better, I'll let you know ;)