Shimano 8sp STI is compatible with Shimano 7sp, at least for the rear. Sora is the only 8sp group still in production, but you can pick up older Ultegra or 105 levers on eBay. But since STIs do develop slop over time, you may be better off with Sora.
The question in my mind would be why your rear isn't indexing well now. Indexed downtube shifters are dead simple and don't wear out easily. If the shifter itself is still giving you positive clicks, the problem is more likely in the adjustment, the cable, and possibly the rear derailleur.