RPM does a nice little set for very little money:
http://www.squadron.com/ItemDetails.asp?item=MKR35901
There are several photo etched sets that add immensely to the 7TP kits. The two basic kits are the Mirage kit and the Spojnia 7TP. The Spojnia kit is older and the Mirage kit is better. Most of the detail sets are geared towards the Mirage kit. They also expanded the line and put out Soviet variations of the 7TP like the T-26. Also search for items geared towards the Vickers 6 ton tank F and E as the T-26 and 7TP were both based off of this British design.
Airwaves did a PE set for the Spojnia kit. It was 35021 and if you ran across one, it would be luck. Yeah, I have one, but I'm lucky. Again, try searching for T-26 detail sets and you will be all right.