If your tack items were the only things stolen, I'd think it was an inside job. Do you have any enemies among the rest of the boarders? People can be petty and vindictive, and horse people are some of the worst.
I do find it a little hard to believe that a thief would take ONLY your stuff, especially since by your own admission it's not top of the line. If they were going for things they could resell, I'd think they'd want to take the better tack.
I'm a little surprised that your own grandmother won't help you out, as well as the BO. If they have tack they're not using, I don't see why it can't be loaned to you on a short term basis until you can replace yours.
I've always kept my tack at home, because there are thieves and 'borrowers' everywhere. I do suggest once you replace your items, that you buy a locking tack trunk or keep them at home.
Tack is expensive, but there are plenty of places to buy used items for not a lot of money. Good luck.