Go to pricewatch for all your computer needs. It's the first place I look.
http://www.pricewatch.com/

Why do you only want to get one compatible with the Compaq computer? Why not get the exact replacement part to ensure you are not going to have other problems like connectors and wiring.

Call the HP Tech Support number and tell them you want to buy a new power supply. They will connect you to the the proper office and the person will ensure you get the correct part. Be sure to have all the technical data posted on your computer or in the manual.

This is what I do. I purchased 1GB of memory from another source and it didn't work. I quickly learned that there was 8 data factors about memory that had to match exactly in order for it to work properly. I called HP and for an extra $27 (cost of part and overnight shipping) I had the proper RAM and it worked properly as soon as I installed it.

The power supply went out on my daughters Compaq desktop and she tried 3 different power supplies purchased at stores in her town. The problem was the wiring and connectors didn't match. The stores kept telling her that her computer was outdated and the power supply was no longer available. Well, I called HP and the next day my daughter had the exact Power Supply for her computer.

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00838772&lc=e​n&cc=es&dlc=es&product=3352589&lang=es

http://h10025.www1.hp.com/ewfrf/wc/softwareList?os=2093&lc=en&​cc=us&dlc=en&product=3352589&lang=enhttp://h10061.www1.hp.​com/ccsearch/search?pname=Compaq+Presario+SR2173WM+Desktop+PC&pid=​3352587&productBigSeriesOid=3239109&spid=3352589&opname=Co​mpaq+Presario+SR2173WM+Desktop+PC&contextual_lname=&pname_desc​=Compaq+Presario+SR2173WM+Desktop+PC&ptype=consumer&stype=pn​&spname=Compaq+Presario+SR2173WM+Desktop+PC&temp_hql=s-001%2Cs​-002%2Cs-004%2Cs-003%2Cs-005%2Cs-006%2Cs-017%2Cz-001&ctry=us&l​c=en&dlc=en&mode=lz&tmv=0&method=prodFinder&qry=po​wer+supply&Submit.x=4&Submit.y=4or