I have a handful of suggestions, AKA what I would do in the same situation:
1. Check the dumpsters outside of sporting goods stores. I've found many pairs of like-new shoes this way.
2. Military surplus stores can be a good source of boots, but they might be a bit higher priced than funds allow.
3. Check with some of the local churches. I know that a lot of churches get donations of clothing and shoes for the homeless, so this option might work, AND they won't charge you anything for 'em.
4. Check the trash cans in a Walmart parking lot. Last time I did that, I shit you not, I found a pair of lightly used Red Wings boots. They were soaked, but a few days of drying them out, and they were good to go. These are the boots that I'm currently wearing.
5. If the other options don't work, doing the switcheroo at Walmart is actually SUPER simple, because no one is going to question you trying on different pairs of boots, and walking up & down a few aisles to make sure they fit properly. Everyone (?) Does this, I'd imagine.
5.2. If you choose to boost the boots from Walmart, choose which boots you want, and then try on several different pairs, starting with any, and ending with the ones you actually want. When doing this, try to dress like a yuppie if you have the ability, and don't bring all your gear into the store with you. Put your old shoes into the box the new ones came out of, and as long as no employees actually SEE you do this, or follow you, you can go right to the checkout and buy something small, like a drink or magic cards or something, and then walk right out, because many people these days will NOT notice what shoes a person is wearing.
Don't appear or act nervous. Carry yourself as if you know EXACTLY what you're doing, and as if the shoes were already on your feet.
If you get stopped at the door, just fuckin' book it. I don't think Walmart employees are allowed to chase shoplifters out of the store. (I'll check on that for ya.) Plus, you'll have on some new shoes, so you'll have an advantage, Lol!
Hope this helps!