PSU's are quite cheap specially if you only need a 400W PSU (Most computer stores won't even sell that low anymore. Lowest you will usually find is 500+). So you could probably pick one up for a extra $40 - $70 if you watch for sales on them.
Also I must stress when buying a PSU and definitely a PSU is make sure you are getting it from a reputable maker. Not all companies sell the same quality of GPU and PSU's. Some are very reliable and others tend to break often. So just make sure to do the research before buying.
As for a PSU I would recommend maybe this http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028
It is a 600W PSU and Corsair tends to be very reliable when it comes to PSU's. It is actually quite cheap also ($79.99 normally but has a instant savings of $10, a promo code of $10 off and a mail in rebate of $20.00. So basically after all that you are paying $39.99 for the PSU which is quite cheap).