i would like to know if there are compatible 3rd-party video card heatsink-fans around that i could use to have a better cooling for my Voodoo3 cards?

I noticed most of the heatsinks i have here doesn't have the same pin holes locations, so i can't use those on them.

Or is it better to leave the "glued" heatsink on and just use a PCI slot blower or something to get rid of the heat "floating around the card"? or case fans are just enough?

what I used to do with my voodoo3 cards, was just screw on a 4 or 6cm fan and let that cool the cards.

It screws up the image of the card, but it will keep it cool.

If that is not an option for you, you can always add one of those pci slotbracket coolers and place it next to the card and leave the card unchanged them.
PCI slot cooler does tend to have better airflow cause of it';s size.

A side case fan would be an excelent choice, it will cool the entire card, if you check you'll see that the backside of the card will also get hot (where the V3 "gpu" is located).

the cheap cases i have here doesn't have mounting holes for a side case fan, they are mostly old ATX generic cases. So they use 80mm fans, front and rear. Except for my other case where my Pentium 200 MMX is, it use a 80mm front, and 60mm on the back i think, which is an awkward and uncommon size, i only had 1 of those here.
Since they tend to ne noisy fans at 12V, i just use the 5V and Ground wires on the connector, and using them on 3-pins connectors (12V, 5V and Ground) so i can choose which voltage directly on the board.