Online you won't find anything completely accurate because the colour on the screen is made with light rather than inks and pigments.
Good quality RAL reference books are made by paint firms and the swatches are actually small pieces of paint product so they are perfect (as perfect as perfect can be)... these are expensive... often in the hundreds of pounds.
The official RAL books are created with ink rather than paint so they're not always spot-on but they're what RAL sells so they're accurate as far as they're concerned.
You can get one from Amazon here - https://amzn.to/2TSa6Tt
Light will fade the colours so store it in a dark, dry place.
There is a greater spread of khaki/drab style colours in the British Standard range than the RAL. From what I understand, these are the colours that are used in official camouflages and such.
You'll also find that some shops will have tube samples of colours but this probably isn't the best reflection because it only shows what each RAL will look like when applied by one shop in certain circumstances and there can still be variables.
The reference books show what the colours are "supposed" to look like when applied well.
This can go wrong if you don't groundcoat properly... if you don't stir the paint properly in the tin... if it's too warm... too cold etc...
In terms of working to a budget, there are some trade-offs... one of those is simply that the colour range isn't available... it's one of the reasons people opt for wet-spray refinishing in order to get the complex colour variants and designs possible.
If you're London based you're welcome to come and visit our workshop and flick through the reference materials here... we've got them all... it's actually too many and an easy way to waste your morning and not realise. Drop me a DM.