Hey There, I completely understand your dilemma---I recently had to move to a location where there are no boarding facilities to complete my degree and will be here for a year, so I am in the exact same boat as you! My horse is my baby and I have had him forever! Leasing is definitely the best option and what I am doing now. The more flexibility you have with the lease, the better your chances of leasing are and by that I mean being willing to let him go offsite, being flexible in how much the leasee pays, etc. Things to consider---would they have to stay in the same city or state as where they are now? Do they need certain amenities, etc?
Also, I don't know about you, but something that became tricky for me was that I am adament about meeting the potential leasee, watching them ride, etc. which obviously becomes a problem when you're on the other side of the country---so advertise the lease well in advance of your move as it may take a while, be prepared to travel back and forth as needed to show your horses to potential leasees or if travel is not an option, ask someone you trust to takeover and "screen" leasees when you're gone.
As for terms of the lease, some folks ask for only board, some ask for board and shoes, other for board, shoes and routine vet bills. In general, non-routine vet bills are always paid by the owner, but it's really up to what you want. One thing to BE SURE TO HAVE IS A WRITTEN CONTRACT with any leasee. This can specify the length of the lease (some folks to month to month but require 30 ays notice to terminate the lease, others do a set time period), the price, when you expect to be paid (say by the 5th of each month) and any other terms of the lease (for example if you have rules, I.e. I require that the leasee take lessons on the horse and specify that they are not allowed to jump without a trainer---really any rule you want can be put in the lease). The contract also covers you legally, should the person be injured by your horse (say a fall) OR should the person neglect of somehow cause an injury to your horse. You can find contracts like this online for free if you search around.
Lastly, something else I have also relied on in the past are friends or trainers whom I trust to be dependable and I think will enjoy my horse and I just ask that they ride them and look after them for free (or sometimes I've even paid them to ride them so many days per week). I have been amazed at the willingness of friends to take my horses on when I have had to be away for schooling (and I am talking months here and my horse was being ridden 4-5 days a week). If you are JUST needing them cared for, perhaps you could even advertise that you need someone to exercise your horses---they ride for free in exchange for a free ride. Of course, sometimes this results in folks loosing interest or not being as dependable, but it could be better than nothing.
Lastly, are you sure you can't take your horses with you? If you can afford to move them, it may be worth it. Perhaps you can't know about the facilities out where you will be, but perhaps you can get there, figure out your finances and familiarize yourself with the horse facilities out there and found a good home for them where you are. I have spent a decent amount of money transporting my horse with me to school, home on breaks, etc. but it was worth it to me and I was lucky enough to be able to afford it.