I'm surprised the beer is flat. Transferring from keg to bottle can cause some carbonation to be lost, but the beer is rarely completely flat. Do you know if the snap-tops are air tight? If they're not, then you'll of course need to find some other suitable bottles. Screw-top bottles are air tight, and readily available.
If you don't have access to a CO2 tank, then you could try adding some priming sugar. If the beer is very clear, then there probably is not enough yeast to ferment the priming sugar, and you should also pitch some yeast too.
When working with priming sugar, you might be thinking of adding a little to each bottle. However, it's usually more accurate and sanitary to weigh out the entire amount (ca. 8oz/200g) and add this to a small quantity of water (say 5oz/130 ml), which you then boil and cool. One teaspoon of solution is then the right amount for each bottle. If you need to pitch some yeast, pitch about 1g of dried yeast into the cooled sugar solution (pitch at 75 °F). That will be sufficient to prime 25 liters.
If you have access to a CO2 tank, you could force carbonate by using a "carbonator" lid that screws on to plastic soda bottles.
At $15 a piece, these are quite pricey. But you can make your own from the existing plastic bottle cap and a tire valve stem, as described here.
This can help save your current batch, and will certainly be useful in future if you ever get more beer kegs you want to bottle.