He likes to sit at the little table that we got him and play with his play-doh or read his books, but the chair that we had bought to go with the table just wasn't working for him.
Enter my brilliant husband...
He found this little wooden chair at IKEA and added the shelf braces, also from IKEA, to provide the perfect little chair for Isaiah. The straight back helps him sit straighter and not slide. The horizontal slats on the seat seem to provide some resistance to sliding as well.
wooden Ekby Valter bracket, $4 in the States, but turned differently on our chair
Unfortunately, the chair isn't pictured online without the brackets, and I forgot to get a picture of the chair before my husband's improvements. The chair cost about $11 with our current exchange rate, so it's probably cheaper in the States. Altogether, with screws and everything, we were able to have this for under $16. (My husband wanted me to say that he already owned the drill.) :)
Yard sales aren't that common over here, but you might be able to find something similar for even cheaper if you're able to shop thrift stores or yard sales.
Isaiah sat in his chair for an hour today without sliding or having to be adjusted--great for both of us!