The following info comes from someone who has never even handled an AR 15 - with that in mind:
Bushmaster or Olympic Arms should sell 7.62 x 39 uppers.
In my experience, these are harder to find than .223 uppers, but not like hens teeth.
No special mods required to lower - you're not going to use an AK mag. You're going to use a 7.62x39 AR mag. (If you go this route)
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If you're looking for other rifles in 7.62x39, you could:
Get an enfield re-chambered
Get an Arisaka re-chambered
Either of these options should be possible for under $250, inclusive of purchase of rifle
Get other AK rifles. Why not? Set them up with different furniture, different sighting options, fuzzy dice....
Get a CZ 52 (Not entirely sure on model - this was a rifle chambered for 7.62 x 45 . Then re-chambered and worked over to use x39)
Get a Mini 30.
Get a Rashid carbine. These are somewhat rare in the USA. Never heard of one? I told you it was rare. It's a 7.62x39 version of the Ljungman (some nordic country, it's been a long day) and the Hakim (8mm - used briefly by Egypt. A complete BLAST to shoot. No recoil). These are semi-auto rifles.
Get a ruger 77 in 7.62x39. They made a limiited run a few years back.
I sincerely hope that this has been of some use to you in your quest. For what it's worth, you will have better luck asking this question on the reformed ak-47.net