Bob, I went to several sites . I needed to make sure the element was the right size and shape, and that the terminals did not stick out too far from the tabs that screw into the oven - for that http://www.repairclinic.com/
is great since they have a scale photo of nearly every element. Unfortunately, they don't usually list wattage. Here is one site with diagrams and wattage listings http://www.hescoapplianceparts.com/pdfs/bake_broil.pdf
At this site, once you go to stove, oven parts, you can filter by length http://www.partselect.com/PartSearchWizard.aspx?Appliance=Range
Here is a catalog, start at page 460 http://188.8.131.52/wda_cat.htm
They also have a webpage for locating elements http://www.wdarmstrong.com/BandB/html/BandB.html
starting at page 6 there are diagrams with wattage and dimensions.
There was another site, I can't find right now but will look for, that had an old online catalog of elements listing rp numbers, and wattage and voltage for many elements. UPDATED I found it
Supco is the one, this is their 2012 version http://www.supco.com/images/Appliance%20Catalog/120-145%20Final%2012-18-06.pdf
scroll down to page 135 to find bake and broil elements, The Supco main page is here http://www.supco.com/
BTW, most of the vendors are set up to sell by model number of the oven or range, but usually list the manufacturer number as well - which often starts RP, CH or WB, and sometimes there are several manufacturer numbers for the same part -
Once you have the RP or CH or WB number , you can also check other vendors to see if they have it in stock http://www.partshousesupply.com/servlet/the-2588/Y07431100-Range-Upper-Broil/Detail
or even do a google search of that number, it was not uncommon for one vendor to say item was in stock at $60 another to list it at $25, and have 2 or three say it was discontinued and no longer available, so it pays to check around. If you are converting from screw on to spade connector, you will also want to buy heat rated connectors. The screws typically common with the element, but not always. Good luck. If you find some other vendors, post them here so the next guy will have it easier to find than me. UPDATED, don't forget that you have to be able to get the part inside the oven, I found one or two that looked like they would be just the right size, but then realized that with the length after the tab, there was no way I could get it into the oven.
As always, when playing with electricity, play very safe. If you increase the wattage, make sure that everything from the outlet to the element can handle it - that includes every switch, thermostat, wire etc. The switches and thermostats are usually labeled with max watts, for the wire, if you can find a bare section, use a caliper and measure the diameter, then compare it to a wiring chart to make sure it is the appropriate size, if in doubt upgrade. All wiring,, connectors, and terminals should be high temp rated, and you can get them at repair clinic, though I did have trouble locating wires with different colored insulation. I didn't change out the thermostats, so I know that the oven assembly and insulation would be fine, and suggest that if you need to upgrade the thermostat, you get one with the same upper limit on temp as one you replace. Good luck.