You can get the URA3 dna sequence from the yeast genome bank.
I can get you the URL if you need it.
I'm trying to knock-out a gene with URA3 too, and I created a knock-out
construct which has URA3 replacing the entire coding region of MFG (ATG
of my gene replaced with ATG of URA3). I also included about 100-150
bp of 5' and 3' flanking regions of MFG (to include regulatory
sequences, etc.). I got the construct made and am trying to transform
yeast with it .... so far no luck , although this is in progress...
So let me know what you find out, and what works..!!!