You'll likely want a 2ohm speaker to continue to get the most out of the stock amp, but those are few and far between, especially in 8" size. Finding one with a depth that doesn't hit the window when it's down will be even more of a challenge.
I had a set of Infinity 60csx components up front that had nice midbass. They use 2 ohm components, though you'll need an adapter for any non-OEM speaker and that will drive up the cost. Still a lot cheaper than LH, however since you can opt for just a pair.
If you can't find a used 8" replacement, I'd probably buy a set of OEM 6.5" drivers and put them in the back, then use one of the 8" drivers from the rear in the front. The rear speakers are tuned to be mostly useless anyway. The 6.5" and 8" speakers share the same mounting holes so they are interchangable.