You won’t find a ready to use e-foil board for less then $1,000-$2,000.
If you don’t want to build one, and don’t have the budget to buy one I suggest just attaching a waterproof box to the top of a already made foil board, or SUP foil board. Even these cost over $1,000. But, you might be able to score an old used one cheaper.
Really, IMHO… The cheapest board build method I have seen is using an old windsurfing board. There are several builds like this on the forum. You can get them very cheap. Just cut the tail off and use the nose section. Pretty easy to fiberglass the cut tail. And pretty easy to add a foil mount “track box”. You can buy the track box already made, in a block of high density foam. Just cut a hole and epoxy it in, fiberglass over. For electronics use a waterproof box on the top of the board if you don’t want to do any extra board work.