I just picked up all of my fertilizer for the year from Golden Furrow fertilizer in Agency, IA (I think they have locations all over the state). They are able to mix you up whatever mixture of N-P-K that you need (I would suggest getting a soil test so you don't have to buy more then what you need) and were very reasonably priced.
Also, I 2nd the importance of lime. Acidic soil will prevent your clover from utilizing the fertilizer that you put down. Lime is extremely cheap compared to fertilizer...