Can i fit a tow bar on my peugeot 309 on my own or do i need specialist help and how much would it cost

They usually fit it for free, about £150 is the average price UK.

l believe the frame of the 309 is exactly the same as a 205. l have 3 205 is & one of them has a tow bar.
You can install it yourself with bolts of 10 mm, & lock washers. The holes were already there in my 205 frame.
Removing the rear bumper will be necessary, have some penetrating oil handy for those rusty bolts from the 309.1985 - 1997
l have seen some with two arms to attach to the frame in two places, & l have seen the one-arm type. Both attatch to the same center bolts under the bumper as well.
l do not think u will have to buy the towbar as new, but might find one cheaply as ''used''. Removing it from an old car will help u see how it was attached. However l recommend buying NEW wiring & the 'plug-in' to have no old corrosion.

The wiring can be spliced into the tailight wires, which r easily found inside the boot area.

Do a good job of splicing the wires, especially the GROUND wire, (directly to the body metal) because all ur new trailer lights will draw a lot of amps.

Fitting a tow bar is not too difficult, I've fitted a few over the years on a range of vehicles. l suspect from the way ur question is phrased that u r not an enthusiastic or regular amateur mechanic, so l would suggest that u may want to have it professionally fitted. Most places now will give u a combined quote to supply & fit, & to be honest, if it is a quick bolt on job as opposed to one of the more complex bars, then for the little it will cost to have it fitted, it is not worth doing it yourself. You will also have the peace of mind knowing that it is been done properly.